Across:
8 Course (“course” = “coarse” = VULGAR) on the radio (= homophone indicator) is tacky (VULGAR),
9 Show up at (show up at = SOUTHPAW) play (= anagram indicator) by lightweight (boxer) proponent of The Left (SOUTHPAW),
10 Questions (ASKS) things to do (tasks) off the top of the head (t) (‘tasks’ without ‘t’ = ASKS),
11 What made the Ali shuffle possible (FEET OF CLAY = feet of Cassius Clay/Muhammad Ali) is a fundamental flaw (FEET OF CLAY),
12 High-flyer (hawk) sacrifices weekend (‘week’ end = k) (‘hawk’ without ‘k’ = HAW-) at first-class (-AI-) institute (-I) (HAWAII) in Pacific Ocean (HAWAII),
14 Sounds like (= homophone indicator) somewhere in West Cork (“Schull” = “skull” = SKULL -) and Cork (cork/lid = -CAP) (SKULL CAP) for one of the family associated with The Mint (SKULL CAP is from the mint herb family),
15 One of The Stones (“jewel” = “dual” = DUAL) spoke out (= homophone indicator) to give the subject some recognition (CITZENSHIP) (DUAL CITZENSHIP) for those calling home in Europe and America? (DUAL CITIZENSHIP),
18 Orpheus’ (Orpheus = PUSHO-ER) adaptation (= anagram indicator) about victor (-V-) (PUSHOVER) or unworthy opponent (PUSHOVER),
20 Guard (ENSURE), engineer (E-), nurse (nurse = -NSURE), butcher? (= anagram indicator) (ENSURE),
22 Is that not repellent (CITRONELLA) to burst (= anagram indicator) into cellar? (into cellar = CITRONELLA),
24 Leave port (SAIL) on banks of Santa Fe Trail (on banks of ‘Santa Fe Trail’ = SAIL),
25 Snug tent (snug tent = TUNGSTEN) made (= anagram indicator) with very hard material (TUNGSTEN),
26 Friend of dancer (reindeer DASHER friend of reindeer Dancer) is a fast mover (DASHER).
Down:
1 Dilettante leaves industrial estate (‘dilettante’ leaves ‘industrial estate’ = usrisa = RUSSIA) in Eastern Europe (RUSSIA),
2 Tried to abandon tragedies (“tried” to abandon “tragedies” = ages = AGES) for a long time (AGES),
3 The writing is on the wall (GRAFFITI) and it can be cutting (cutting/graft = GRAF-T-) about endless (x) drug taking (‘fix’ without ‘x’ = -FI-) at institute (-I) (GRAFFITI),
4 Clare, for one (Clare Island = ISLE), immersed in tennis lesson (immersed in ‘tennis lesson’ = ISLE),
5 You and I (us = SU-) review (= reversal indicator) party (-DO-) for returning (= reversal indicator) neighbours (UK = -KU) (SUDOKU) – the difficulty is getting the numbers right (SUDOKU),
6 They’re reminders of what’s still to be done (CHECKLISTS) as European (“Czech” = “check” = CHECK-) state (= homophone indicator) lost (-L-) to winners (1sts = -ISTS) (CHECKLISTS),
7 Retiring (= reversal indicator) actor – the bad type (ham = MAH-) from America (-A-) campaigned (-RAN-) with single (-I) (MAHARANI) by Queen in the East (MAHARANI),
13 Every ounce of anger (ALL THE RAGE) in Vogue (in vogue = ALL THE RAGE),
14 School (S-) and institute (-I-) in Kildare (KE = -K-E-) is right (-R) (SKIER) for one with a sporting career that’s going downhill (SKIER),
16 Takes a lot of interest (lending at high/extortionate interest rates = USURIOUS) in you and me (us) on both ends (us + us = US-US) of centre court (centre ‘court’ = -U-) in Brazil (-RIO-) (USURIOUS),
17 Criminal (= anagram indicator) leaders (leaders = E-ERALDS) seizing one thousand (-M-) (EMERALDS) green gems (EMERALDS),
19 Narcissism (VANITY) of one (-I-) from the capital (‘the’ capital = -T-) in navy (navy = VAN-Y) dress (= anagram indicator) (VANITY),
21 Made a surprise attack (RAIDED) to remove spice from rice paddies (remove ‘spice’ from ‘rice paddies’ = raddie = RAIDED),
23 Lorraine lets out roar (‘Lorraine’ lets out ‘roar’ = line = LINE) in bar (LINE),
24 Something my father wore (‘The Sash My Father Wore’ = SASH) in Africa? (South Africa = SA-) Keep that under your hat! (-SH) (SASH).