Bartok perhaps eating sides off enormous whale (6)
I believe the answer is:
beluga
'whale' is the definition.
(beluga is a kind of whale)
'bartok perhaps eating sides off enormous' is the wordplay.
'bartok perhaps' becomes 'bela' (composer Bela Bartok).
'eating' is an insertion indicator.
'sides off' says to take the centre.
'enormous' becomes 'huge' ('huge' can be a synonym of 'enormous').
The central letters of 'huge' are 'ug'.
'bela' placed around 'ug' is 'BELUGA'.
(Other definitions for beluga that I've seen before include "Large sturgeon, source of ..." , "fish" , "Great Russian sturgeon - Arctic whale" , "Small northern white whale" , "Species of sturgeon" .)