Deny slice of fine gateau (6)
I believe the answer is:
negate
'deny' is the definition.
(negate can mean to deny or refute)
'slice of fine gateau' is the wordplay.
'slice of' indicates the answer is hidden within the clue.
'NEGATE' can be found hidden inside 'fine gateau'.
(Other definitions for negate that I've seen before include "Show to be false" , "Cause to have no effect" , "Age ten (anag)" , "Render ineffective" , "Opera by Puccini" .)