The presence of two recondite words at 4 across and 4 down means that this puzzle is going to find a few out - including me. I 'resorted to aids', as the saying goes, pleased to have the excuse of needing to post this blog to put me out of my misery. 26 ac also brought me up short - so all in all not my finest hour.
Across
1 ASIA - reverse hidden
4 SCHIPPERKE - CHIPPER inside S + K[or]E[an]; something to add to the list of famous Belgians?
9 THORN APPLE - hopplanter*; a prototypical 'weed' with white trumpety flowers and prickly leaves, called jimson weed stateside.
10 omitted - even those who wrote this straight in are probably not feeling especially this.
11 NOGGIN - N + going*; a snifter is a drink peculiar to 'stage colonels' such as Jimmy in Reggie Perrin and the major in Fawlty Towers.
12 AIR+CRAFT
14 CHAT - orwight my son? 'chat up' is slang for addressing a lady with a view to a conquest, 'bird' is slang for a young lady, and a 'chat' is a warbler.
15 COASTGUARD - runners as in smugglers.
17 GUIDELINES - GI around the bra of the social etiquette world (U) + DELIS around NE.
20 PS+ST - a post-script is a rider, as in a proviso or supplementary clause.
21 READ+JUST - I had to get this from the wordplay as I am unfamiliar with Wallace's stuff; the book is The Four Just Men.
23 RHEBOK - herb* + OK; very much the bra of the antelope world at the minute, albeit in a different guise; watch out for 'rhebuck', which also pounds the African plains.
24 BOOM - double definition.
25 INORDINATE - the literal is easy enough, but the clueing is rarther canny: insubordinate minus the sub (editor).
26 SLEIGH BELL - Dancer is one of the bullies who became toadies after Rudolph's elevation by Santa. A parable for our times.
27 omitted
Down
2 SCHOOLHOUSE - SCHOOL + H + OUSE; a fair few solvers will have been working around Peterhouse, Porterhouse, etc. and picking their brains for a specific college or uni.
3 ARROGATED - Harrogate minus H and plus D.
4 STANNIC - tincans*; I tries making anagrams from that and cansnot but all to no avail. Do I hear chuckles from Barsetshire?
5 HOPE AGAINST HOPE - title of a remarkable memoir by Nadezhda Mandelstam.
6 PIERROT - ERRO[r] in PIT; I happened to watch an episode of Men Behaving Badly in which the bloke who isn't Martin Clunes was hamming it up as one of these characters, white gloves an' all.
7 R+U+MBA - some won't care for the airy-fairy literal 'one from Cuba', but since this is the bra of the Caribbean cruciverbal terpsichorean world I for one am cutting the setter some slack.
8 EIGHT - double definition: oarsmen and one way of saying the past tense of 'eat'; I'm pretty sure I say both, though not at the same time.
13 FORESHOR[T]E+N[ot]
16 UMPTEENTH - putthemen*
18 LOUT[IS]H
19 STRUDEL - D in ulster*
21 REBUS - RE + sub reversed
22 ABODE - A+BOD+E, where the bloke must come before the E[uropean].