Thank you to Pangakupu (whose Cryptic, by the way, I blogged last Thursday). Definitions are underlined in the clues.
Across
1. One revealing a secret story of archery and apple? (8)
TELLTALE : [ TELL TALE](a story/tale of William Tell, the legendary Swiss expert with a crossbow/an archer who shot an apple off his son’s head).
5. College grounds affected our group (6)
CAMPUS : CAMP(affected/put on) + US(pronoun for “our group”, in contrast to “them”).
9. Rugby player with three legs? (3,4)
FLY HALF : Cryptic defn: A fly with three out of the usual six legs, may be so described.
10. OAP, perhaps, regarding one in tree (7)
RETIREE : RE(regarding/about) + [ I(Roman numeral for “one”) contained in(in) TREE].
Defn: An Old Age Pensioner/… could be one.
11. Still time for an occasion (5)
EVENT : EVEN(still, when used to emphasise, as in “he’s better even than his teacher”) + T(abbrev. for “time”).
12. Royal Household drugged? That’s where judges will be needed (4,5)
HIGH COURT : [ HIGH COURT ](possibly, a royal household/court that is on drugs/high).
13. Stop – when the pepper runs out? (5,2,1,4)
GRIND TO A HALT : Cryptic defn: What happens when the pepper mill runs out of peppercorns to grind.
17. Long sentence with full stop? It’s critical (4-3-5)
LIFE-AND-DEATH : LIFE(a long prison sentence) AND(with) DEATH(a full stop/absolute end).
Defn: …/vitally important.
20. Explosive set to catch fire (9)
GELIGNITE : GE(of a liquid or semi-liquid substance, to set/to become more solid) + IGNITE(to catch fire/to set alight).
22. Church in America or another country (5)
CHINA : CH(abbrev. for “church”) + IN + A(abbrev. for “America”).
23. Excessively quote old poetry about Unionist (7)
OVERUSE : O(abbrev. for “old”) + VERSE(poetry) containing(about) U(abbrev. for “Unionist”).
24. Recalled uproar about limits to our coastal decline? (7)
EROSION : Reversal of(Recalled) NOISE(uproar/din) containing(about) 1st and last letters of(limits to) “our”.
Defn: Action of wearing away/decline, in this case of the coast.

25. What helps to regulate the Rottweiler? (6)
TETHER : Hidden in(What helps to) “regulate the Rottweiler”.
Defn: Chain or rope to restrict/regulate an animal’s movement, in this case a Rottweiler dog, reputedly an aggressive breed.
Not sure of the indicator for “hidden”.
26. Man and dude confused about Democrat’s extra comment (8)
ADDENDUM : Anagram of(… confused) [MAN plus(and) DUDE] containing(about) D(abbrev. for a member of the Democratic Party).
Down
1. Not working to block part of golf-course? That’s sweet (6)
TOFFEE : OFF(not working/having a rest day) contained in(to block) TEE(part of a golf-course at the start of each hole).
2. Bet list of options getting reduced for ordinary fellows (6)
LAYMEN : LAY(to bet/to stake money in a bet) + “menu”(a list of options, for food in a restaurant, say) minus its last letter(getting reduced).
Defn: … without professional or specialised knowledge in a particular subject.
3. Italian restaurant dessert overturned and a riot breaks out (9)
TRATTORIA : Reversal of(… overturned, in a down clue) TART(a dessert/sweet course at the end of a meal) plus(and) anagram of(… breaks out) A RIOT.
4. Sinister ambition as seen in US auto industry products? (4-4,5)
LEFT-HAND DRIVE : LEFT–HAND(sinister/to the left side) DRIVE(ambition/determination to achieve).
Defn: …, ie. cars with the steering wheel on the left side for driving in countries, the US included, where traffic flows on the right hand side of the road.
6. High level to be attained in Greek? (5)
ATTIC : Double defn: 1st: The highest space or room of a house, located immediately under its roof; and 2nd: The dialect of Ancient Greece/Attica.
7. Left capital of Uruguay with girl, coming to European country (8)
PORTUGAL : PORT(the left side of a ship or aircraft facing forward) + 1st letter of(capital of) “Uruguay” plus(with) GAL(an informal American term for a girl).
8. Watch indecent tapes etc. (8)
SPECTATE : Anagram of(indecent/improper) TAPES ETC.
10. Senior cleric correct to turn back almost before conclusion (5,8)
RIGHT REVEREND : RIGHT(correct/true) + “revert”(to turn back/return to a former, say, state) minus its last letter(almost) placed above(before, in a down clue) END(conclusion/finish).
Defn: Form of address for a ….
14. Vegetable lorry that guy brought round OK (9)
ARTICHOKE : ARTIC(an articulated lorry) + HE(pronoun for “that guy”) containing(brought round) OK.
Raw:
and steamed: 
15. Initially locked in lavatory – situation displayed in online comment (8)
BLOGPOST : 1st letter of(Initially) “locked” contained in(in) BOG(slang for a lavatory) + POST(a job/situation, as in the “situations wanted” ads in newspapers).
16. Wealthy area heading off industrial waste (8)
AFFLUENT : A(abbrev. for “area”) + 1st letter deleted from(heading off) “effluent”(waste discharged, in this case, from industrial plants).
18. Felt sad for coalmine that is dead (6)
PITIED : PIT(a coalmine) + IE(abbrev. for “id est”/that is) + D(abbrev. for “dead”).
19. Magazine number showing quantity of champagne (6)
MAGNUM : MAG(short for “magazine”) + NUM(abbrev. for “number”).
Defn: … in a bottle of 1.5 litres capacity.
21. Assess Georgia – not initially vast (5)
GAUGE : GA(abbrev. for the US state of Georgia) + 1st letter deleted from(not initially) “huge”(vast/enormous).
A great quiptic, easy and yet thoroughly enjoyable; not too many anagrams and hilarious wordplay. I liked all the long entries, especially the reference to the Latin meaning of “sinister”. Also liked the rugby player with three legs. Thanks Pangakupu and scchua!
Thanks Pangakupu and scchua!
Nice quiptic. Great blog as usual.
TETHER
‘What helps’ seems to be the intended ‘hidden’ indicator.
Amazed at the skill of Pangakupu – beautiful quiptic today; almost bewildering cryptic earlier in the week. FLY HALF, TELL TALE and GRIND brought a smile and TETHER took a while to find – was the question mark enough to show it was embedded? (Typed same time as KVa @2)
Thank you scchua and Pangakupu
Quite tough for a Quiptic.
Favourites: GRIND TO A HALT, HIGH COURT.
I could not parse 25ac, 16d although I guessed something like that was going on between affluent/effluent.
New for me: 9ac FLY HALF ( I know zilch about rugby) and I could not parse it either.