Only one of the words was new to me (17ac), but the wordplay left only one possibility, which a quick check on onelook.com confirmed as the right answer.
ACROSS
9 RAISIN – RA(1’s I)N – not sure how a fruit could protect one’s ego – throws up some interesteing images!
10 ENTRANCE – wish I had a pound for evry time I have seen this word clued in this way, or something similar.
11 GUSTAV HOLST – (Volga St thus)* – when compiling, I wouldn’t normally include an abbreviation in the anagram fodder. Would be interested to hear other solvers’ views on this. Surely “Volga St thus…” would have worked just as well.
15 EVICTOR – E-VI-C(T)OR – good surface and clever wordplay.
17 KINGCUP – a swamp plant with buttercup-like flowers
18 UNASSERTIVE – U-NA(<=tress)IVE – I wasn’t sure about the definition “not positive”, but Chambers has positive as a synonym for assertive.
22 RIGADOON – ROG-ADO-ON – a lively dance for one or two couples, depending on which reference you look up.
23 TAG END -TA(GEN)D
DOWN
1 ULTIMA – hidden in “resULT I MAde”, definition is “last syllable”
2 CORN FLAKES – CO(RN Fla = Florida)KES – I’m not convinced by “covering”, but can see how it might mean “on top of”.
3 ALFRETON – A-L-FRET-ON – a town in Derbyshire
4 HERE GOES – (gee horse)* – this seemed a bit contrived to me
7 PINT – PI(N)T – Surface doesn;t make sense and “of volume” as the definition is weak.
12 HEREABOUTS – H(ot) + (soubret(T)e a)*
13 OCCIDENT – homophone of OX + I(DEN)T – the “in” in the clue is superfluous, but necessary to make the surface work.
14 SPLENDID – SP-LEN-DID – “sp” = sine prole, without issue.
21 AGED – (<=DEGA(s)