1 Making Mark proud of himself, impressing duke with drink (8)
SMUDGING - D GIN in SMUG
6 Rubbish musical about Heart of Midlothian (3,3)
HOT AIR - OT [middle letters of midlOThian] in HAIR
9 Journos get shot of Republican parties in affair (2-11)
CO-RESPONDENTS - CORRESPONDENTS (posh journalists) minus one R
10 Window that opens in theatre (6)
LANCET - The first of three clues that all feature this not that commonly seen device (i.e. a clause starting with a determiner - in our cases, 'that' and 'what'); a LANCET is one of those narrow, pointy, often stained-glass, windows in a Gothic church, and also a knife that opens people up in an operating theatre. My last in.
11 Leggy bird, alert, finding similar prey? (8)
CRANEFLY - a cryptic definition that aspires to be an all-in-one? CRANE (leggy bird) FLY (alert), followed by a bit of a leap to get to CRANEFLY, which cranes do eat, apparently, if frogs and fish are in short supply, or if they just want to snack between meals. A CRANEFLY might also be described as leggy if compared with, say, your average centipede.
13 Fool mending faulty carpeting (10)
CONDEMNING - CON (fool - verb) anagram* of MENDING
15 Launch hotel address online (4)
HURL - H URL
16 Beat team driver for one (4)
CLUB - triple definition, with club as in 'Manchester United, the world's greatest club'. One is allowed to dream...
18 Investigator dealing with Home Counties radio serial briefly (10)
RESEARCHER - RE SE ARCHER[s]
21 A trio made a mess of broadcast between planes (3-2-3)
AIR-TO-AIR - A TRIO* AIR (broadcast)
22 Daily, say, sent back fee (6)
CHARGE - CHAR EG reversed
23 Blue Berets merited one story paper put in without reason (13)
UNJUSTIFIABLE - UN (UN peacekeepers wear blue berets - while they can) JUST (merited) I (one) I (the i is arguably Britain's least known newspaper) in FABLE (story)
25 North Sea port offering little work during peak (6)
BERGEN - ERG in BEN; BERGEN (or Bjørgvin) is a port in SW Norway that fancies itself as a bit of a spa, given it's warm(ish), Gulf Stream influenced climate.
26 What invading Spaniards sought in field or a domain (8)
ELDORADO - hidden
2 Miserly old city council imported construction kit (7)
MECCANO - CC in MEAN O; I had both Lego and Meccano as a kid and didn't much care for either. If pushed, I would choose Lego, as it couldn't cut you and what you made had a chance of looking like something.
3 Frank girl coming out with appeal for regular transfer (6,5)
DIRECT DEBIT - DIRECT DEB (girl coming out) IT (sex appeal); a bit of a retro clue. I don't think my uber-woke daughter (whose birthday it was yesterday, which I celebrated by baking an orange and white chocolate sponge) would approve.
4 Chic clique that often appears in magazine (5)
INSET - IN SET; see 10a
5 Service head devouring new Greek character's food (7)
GNOCCHI - now, the service head appears to be part at any rate of GOC in C (General Officer Commanding-in-Chief); which contains N (new) and is followed by CHI (Greek character). GNOCCHI is/are a type of pasta sometimes made from potatoes and sometimes not.
6 Backer mostly into monster plot feature (9)
HYDRANGEA - ANGE[l] in HYDRA; 'plot' as in flower bed
7 Get brown belt (3)
TAN - DD
8 After brief verbal abuse, a caller finally gets cut off (7)
INSULAR - INSUL[t] A [calle]R
12 Ring gong in cleaner, sound lab (4,7)
ECHO CHAMBER - ECHO (ring) MBE (gong, AKA honour) in CHAR (cleaner - again!)
14 It's normal getting thrown out for the likes of adultery (6,3)
MORTAL SIN - IT'S NORMAL*
17 Cash in Europe guaranteed time off (7)
LEISURE - LEI (currency of Romania) SURE
19 Endure nasty virus very close to home (7)
SURVIVE - VIRUS* V (very) [hom]E; our topical clue
20 E for expert? (7)
EGGHEAD - cryptic definition lite, with E being the initial letter of E[xpert]
22 Kid scolded once, having crossed line (5)
CHILD - L in CHID (I think I'd still use 'chid' if I was minded to; Collins suggests it's more common in American English than British, but this type of statement usually starts a spat, so sit back and enjoy the show)
24 What Rose may do, or cat? Not half! (3)
JAG - A rose might jag or cut you, if you didn't observe social distancing rules, and a cat might cut (or scratch) you too, especially (drum roll) if it were a JAG[uar]!! Oh, and see 10a.