Trifles with some fat oyster (4)
I believe the answer is:
toys
'trifles' is the definition.
(I've seen this before)
'with some fat oyster' is the wordplay.
'with' indicates a hidden word.
'TOYS' is hidden in the letters of 'some fat oyster'.
(Other definitions for toys that I've seen before include "Sports" , "Plays (with something), not seriously" , "Children's playthings" , "dolls, for example" , "Trifles with playthings" .)