Unknowingly posh answer -- piercing answer, unfortunately (8)
I believe the answer is:
unawares
'unknowingly' is the definition.
(thesaurus)
'posh answer piercing answer unfortunately' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'answer' becomes 'a' (abbreviation eg in Q and A).
'piercing' is an insertion indicator (some letters pierce a hole in others).
'unfortunately' indicates an anagram.
'answer' anagrammed gives 'nwares'.
'a' put inside 'nwares' is 'nawares'.
'u'+'nawares'='UNAWARES'
(Other definitions for unawares that I've seen before include "Without knowledge of the situation" , "Warn Esau (anag)" , "without intending to" , "Be caught thus to be taken by surprise" , "In ignorance" .)