Grab oysters with youngster (3)
I believe the answer is:
boy
'youngster' is the definition.
(I've seen this in another clue)
'grab oysters with' is the wordplay.
'with' indicates the answer is hidden within the clue.
'BOY' is hidden in the letters of 'grab oysters'.
(Other definitions for boy that I've seen before include "Youthful male" , "Youth" , "Lad" , "Young man" , "Young male" .)