Opera fiends are vacuous fools (4)
I believe the answer is:
oafs
'fools' is the definition.
(I've seen this in another clue)
'opera fiends are vacuous' is the wordplay.
'opera' becomes 'oa' (I can't justify this - if you can you should believe this answer much more).
'are vacuous' suggests removing the centre.
'fiends' with its middle removed is 'fs'.
'oa'+'fs'='OAFS'
Can you help me to learn more?
(Other definitions for oafs that I've seen before include "Dolts" , "Louts on sofa" , "Uncouth buffoons" , "Boors or buffoons" , "Louts, lummoxes" .)