Most oysters come with trifles (4)
I believe the answer is:
toys
'trifles' is the definition.
(I've seen this in another clue)
'most oysters come with' is the wordplay.
'with' says the answer is hidden in the clue.
'TOYS' can be found hidden inside 'most oysters come'.
(Other definitions for toys that I've seen before include "Sports" , "Trifles with playthings" , "Plays (with something), not seriously" , "Fiddles" , "Children's playthings" .)