Completely posh trifle? (3)
I believe the answer is:
sou
'trifle?' is the definition.
(I've seen this in another clue)
'completely posh' is the wordplay.
'completely' becomes 'so' (both can mean 'very much').
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'so'+'u'='SOU'
(Other definitions for sou that I've seen before include "bit of money" , "Old French monetary unit" , "Something of little worth" , "Defunct French coin" , "Tiny amount of money" .)