Thank you to Skinny. Definitions are underlined in the clues.
Across
1. Corrupt villain beginning to ebb into depression (8)
DECADENT : [ CAD(villain/rogue) + 1st letter of(beginning to) “ebb” ] contained in(into) DENT(a depression/a slight hollow in an even surface).
5. Set off stuck in reverse, disheartened (6)
DEPART : Reversal of(… in reverse) TRAPPED(stuck/being unable to extricate oneself) minus its middle letter(disheartened).
Defn: … on a journey, say.
9. Let in or on (8)
ADMITTED : Double defn: 2nd: Let on/disclosed/acknowledged that something is true.
10. Verbally compare fungus with algae (6)
LICHEN : Homophone of(Verbally) “liken”(to compare/to point out the similarities between one thing and another).
Defn: A composite organism of algae living amongst fungi, symbiotically.

12. Amply recover after eruption of bile (9)
LIBERALLY : RALLY(to recover/to get better after, say, a sickness) placed after(after) anagram of(eruption of) BILE.
13. Seconds involving extremely rude customs (5)
MORES : MOS(plural of “mo”, short for “moment”/a second, as in “wait a second/a mo”) containing(involving) 1st and last letters of(extremely) “rude“.
14. Guns, and where one might find them (4)
ARMS : Double defn: 1st: Firearms; and 2nd: Where guns may be found when in use.
16. Authorise adjustment – it lengthens trousers (7)
ENTITLE : Hidden in(… trousers/takes/pockets) “adjustment – it lengthens“.
Defn: …/give someone a legal right to do something.
19. Rainbow iridescence originally seen in various types of mineral (7)
PYRITES : 1st letters, respectively, of(… originally) “Rainbow iridescence” contained in(seen in) anagram of(various) TYPES.
Defn: … containing iron and sulphur.
Aka fool’s gold:

21. Rifle fire (4)
SACK : Double defn: 1st: To steal/plunder; and 2nd: To dismiss from employment.
24. Minor role advanced by job centre (5)
CAMEO : CAME(advanced/moved towards a place) plus(by) middle letter of(… centre) “job“.
Defn: … in a play or film, played by a distinguished actor or celebrity.
Madonna cameo-ing in Die Another Day:

25. Still tense about election’s initial turn out (9)
EVENTUATE : EVEN(still/to a greater degree, as in “it’s still better to be open about it”) + reversal of(… about) TAUT(tense/without any slack) containing(about) 1st letter of(…’s initial) “election“.
27. Graduate managed to drink last drop of rum – he’s often seen in the pub (6)
BARMAN : BA(abbrev. for “Bachelor of Arts”, a graduate) + RAN(managed/controlled an organisation, say) containing(to drink) last letter of(last drop of) “rum“.
28. Maintaining locks in rich area that’s been redeveloped (8)
HAIRCARE : Anagram of(… that’s been redeveloped) RICH AREA.
29. Setter‘s note rejected (6)
RENNET : Reversal of(… rejected) TENNER(a ten-pound banknote/piece of paper money in the UK).
Defn: Substance from calves’ stomachs used to curdle/set milk in cheese-making.
30. Mail a newspaperman returned, at first showing indifference (8)
DEADNESS : Reversal of(… returned) [ SEND(to mail/to forward to) + A + ED(abbrev. for “editor”, a newspaperman/one of the newspaper staff) ] plus(at) 1st letter of(first) “showing“.
Defn: …/unresponsiveness.
Down
1. Twice one’s substituted for United? That’s realistic (6)
DOABLE : “double”(twice/twofold) with “a”(in grammar, an article denoting one in quantity) replacing(…’s substituted for) “u”(abbrev. for “united”).
Defn: …/achievable.
2. Get instruction to sheepdog (4,2)
COME BY : Not sure about this: “come-bye”(an instruction for a sheepdog to go clockwise round the sheep) has no alternative spelling that I can find. Is “get” doing double duty as definition and homophone indicator?
Defn: …/to acquire, as in “how did you come by that thing”.
3. Does taking time prevent… (5)
DETER : DEER(the females of which are does) containing(taking) T(abbrev. for “time”).
4. …a lot of unnecessary drug paraphernalia? (7)
NEEDLES : All but last letter of(a lot of) “needless”(unnecessary/not needed).
6. Imagine fringes of tunic getting altered – it’s inexplicable (9)
ENIGMATIC : Anagram of(… getting altered) [IMAGINE + 1st and last letters of(fringes of) “tunic” ].
7. Observe commotion about woman’s poetry, essentially (6,2)
ADHERE TO : ADO(commotion/a state of fuss) containing(about) [ HER(woman’s/belonging to a woman) + middle 2 letters of(…, essentially) “poetry” ].
Defn: …/to comply with.
8. Heavy metal – it could be symbolised by the introduction of Whitesnake (8)
TUNGSTEN : Cryptic defn: Chemical element whose symbol is/symbolised by the 1st letter of(introduction of) “Whitesnake“.
11. Sort to use 17, amongst others (4)
TYPE : Double defn: 1st: …/kind; and 2nd: To use the space bar/answer to 17 down, and other keys on a keyboard to produce text.
15. Reason to restrict supply of booze (9)
RATIONALE : RATION(to restrict the supply of/to allow only a fixed amount for each person, of a good or service) ALE(beer, an example of booze/alcoholic drinks).
Defn: …/logic behind a course of action or belief.
17. Key location to get a drink that’s out of this world (5,3)
SPACE BAR : SPACE(outer space/out of this world, as an adjective to …) BAR(a location to get a drink).
Defn: … on a computer keyboard, say.
18. Boat to dock beside Irish islands (8)
TRIMARAN : TRIM(to dock/to reduce) plus(beside) ARAN(group of islands off western Ireland).
Star Wars-inspired:

20. Fat creator of Adrian M (4)
SUET : [SUE T]( Sue Townsend, the name of the author and creator of the character, Adrian Mole, similarly shortened as the latter is to Adrian M).
Defn: Hard white … from the kidneys and loins of cattle and sheep.
21. Spot with attendant exudation (7)
SEEPAGE : SEE(to spot/to notice) plus(with) PAGE(a personal attendant to a person of rank, or to a bride at a wedding).
22. Price, say, a lunatic raised (6)
DAMAGE : Reversal of(… raised) [ EG(abbrev. for “exempli gratia”/for example/say) + A + MAD(lunatic/insane) ].
Defn: An informal term for the price/expense to be paid, as in “what’s the damage for this lunch?”
23. Some use elementary uprising for fights (6)
MELEES : Hidden in(Some) reversal of(… uprising) “use elementary“.
26. Bound to eat last bit of cracker that’s stale (5)
TIRED : TIED(bound/restrained) containing(to eat) last letter of(last bit of) “cracker“.
Defn: …/no longer fresh, interesting or exciting.
Thanks scchua and Skinny.
Tricky in places, good puzzle.
In 30a, first ‘S’howing offers the second ‘S’ needed.
Thanks ilippu. Blog corrected.
Good Sunday challenge. Took a while to get going and held up by a few near the end. I parsed COME BY as a double def, without knowing anything about sheepdog commands and now see that it is in Chambers, with the above spelling, as: “(as a command to sheepdogs) move to the left of the group of sheep being herded”.
Probably me being dim, but I wasn’t sure about the ‘where one might find them’ bit of the clue for ARMS. Does it refer to to the upper limbs as holding ‘Guns’ or to a branch of the military or armed forces?
Thanks to Skinny and scchua – you’ve been very busy lately!
Deadness = indifference? Otherwise excellent, so thanks Skinny and Scchua.
COME BY is a straightforward double def for me too; I’m not sure where the homophone idea comes from unless our blogger is thinking ‘Come buy’. But ‘come by’ means ‘get’ as is set out in the blog? Like WP, I am still not sure about ARMS. I don’t see guns as being used by the arms. At best, cradled in them might apply to some guns, I guess.
ADMITTED, DEPART, LICHEN, BARMAN, NEEDLES and TUNGSTEN were my favourites today with SUE T taking the biscuit. Lovely idea.
Thanks Skinny and scchua
Hello all, and thanks to Scchua for the blog – quick note on GUNS – GUNS are well-developed bicep muscles (in Chambers) so they’re going to be found on musclebound arms
Right, bit of gardening, a stroll in the autumn air, and an early night – have a good Sunday everyone
PostMark@5. To explain: as mentioned, I couldn’t find “come by” as the sheepdog instruction. The sheepdog related sites I went into gave “come-bye or come bye”, and I couldn’t find the latter in the dictionaries I accessed. As such, I couldn’t justify it as a double-defn. clue. Then, all I could suggest was that the word-play was a homophone of “come-bye” for “come by”, with “get”/hear/pick up, as the homophone indicator.
Good stuff Skinny, lots to like here including 14&24a plus 8& 20d but my top two the super clever 3&4d.
Many thanks and to scchua for the blog.
Found this one quite a tricky little number in places, wasn’t very convinced by 30a and had a question mark alongside 14a until our setter popped in to explain. My absolute favourite was COME BY – perhaps I’ve just watched too many episodes of One Man and his Dog!
Thanks to Skinny for the challenge and to scchua for the review.
scchua @ 7: thanks for that and, yes, get your point when you make it the second time – which is, admittedly, not that different from the first!
For 20D I was thinking Adrian Monk!
That’s what happens when Americans try British crosswords
I really liked this, but I’m very embarrassed by my entry for 14A (“arms” in French). I didn’t consult the urban dictionary.