Across:
8 For stepping just one foot on the beach (SANDAL), lands a (lands a = SANDAL) flounder (= anagram indicator),
9 Shout it from the rooftops (PROCLAIM) around college (-C-) mostly (d) set in place (‘laid’ without ‘l’ = -LAI-) for seniors dance outside (PRO-M) (PROCLAIM),
10 Tips on back of envelope (tips ‘on back of envelope’ = OBOE) by one of those down the pits (OBOE in orchestra pit),
11 Offers critical advice to the US House (in US, person giving critical advice perhaps = DUTCH UNCLE) from Europe (DUTCH) that’s relative (UNCLE) (DUTCH UNCLE),
12 Anxious (ON EDGE) broadcast (ON) from Trim (trim = EDGE) (ON EDGE),
14 It’s not much to expect (PITTANCE) from old British politician (PITT-) at African party (-ANC-) for English leader (‘English’ leader = -E) (PITTANCE),
15 The Greens are always going on about this (CARBON FOOTPRINT) diamond (CARBON) – it’s a clue for the detective (FOOTPRINT) (CARBON FOOTPRINT),
18 The rugby occasion opens (‘the rugby occasion’ opens = TRO-) for consumers (-USERS) (TROUSERS) covering both legs (TROUSERS),
20 Returns (ELECTS) to select (select = ELECTS) ground (= anagram indicator),
22 Sir (sir = “knight” = NIGHT) heard (= homophone indicator) losing the head (p) with dilemma (‘plight’ without ‘p’ = LIGHT) (NIGHT LIGHT) illuminating aspects of The Dark (NIGHT LIGHT),
24 Produced (“made” = “MAID”) reports (= homophone indicator) with help of the big house (MAID),
25 It’s okay (ALL RIGHT) as there’s nothing left perhaps (ALL RIGHT when nothing is on the left side),
26 On rare occasions (SELDOM) in building (= anagram indicator) models (models = SELDOM).
Down:
1 Opens 15 across (‘carbon footprint’ = CARBON) in type of copy (CARBON copy),
2 David Letterman in part (‘David Letterman’ in part = IDLE) between jobs (IDLE),
3 Slightly give in (budge = B-UDGE-) over fillets, heart (‘fillets’ heart = -L-) and first two onions (first two ‘onions’ = -ON) (BLUDGEON) in batter (BLUDGEON),
4 Content in newspapers potentially (content in ‘newspapers potentially’ = SPOT) is a blemish on one’s character (SPOT),
5 Path to (path to = TOP HAT) adaptation (= anagram indicator) is a formal one for The Crown? (formal one on the crown/head = TOP HAT),
6 Spy (PLANT) from America (A) leaves home perhaps (leaves’ home on a TREE) (PLANT A TREE) to commemorate a visit to The Garden (PLANT A TREE),
7 Kid’s father (kid’s father = BILLY- goat) opens cork and nibbles (opens ‘cork and nibbles’ = -CAN) (BILLYCAN) for the camp dinner (BILLYCAN),
13 A type of humour (D-RY humour) about new (= anagram indicator) Beau (Beau = -EBAU-) with revolutionary (-CHE-) (DEBAUCHERY) type of writing one associates with the Marquis de Sade (DEBAUCHERY),
14 Regularly has a word with the man upstairs (PIOUS) and primary principal (primary ‘principal’ = P =) with debts (IOUs = -IOUS) (PIOUS),
16 Wind up someone (RI-LE) over initial feedback (initial ‘feedback’ = -F-) after (= positional indicator) Express (express/state = AIR) (AIR RIFLE) is responsible for taking a number of pot shots at targets (AIR RIFLE),
17 Nurse (TREAT-) is (-IS-) from Spain (-E) (TREATISE) in the writing study (TREATISE),
19 You’re unlikely to hear this praise if you’re late (EULOGY), as typically it accompanies a put down (EULOGY),
21 Man of the cloth (TAILOR) showing off (= anagram indicator) Rialto (Rialto = TAILOR),
23 Some characters building a terrace (some characters ‘building a terrace’ = GATE) for the players? (GATE theatre),
24 Uncle Sam releases scan (‘Uncle Sam’ releases ‘scan’ = ulem = MULE) of drugs smuggler (MULE).