The middle classes boogie? Sure, I fancy! (11)
I believe the answer is:
bourgeoisie
'the middle classes' is the definition.
(I've seen this before)
'boogie? sure i fancy' is the wordplay.
'fancy' indicates anagramming the letters.
'boogie'+'sure'+'i'='boogiesurei'
'boogiesurei' is an anagram of 'BOURGEOISIE'.
(Other definitions for bourgeoisie that I've seen before include "The middle class" , "Middle classes" , "conventional group" .)