Boy catching large fish (6)
I believe the answer is:
blenny
'fish' is the definition.
(blenny is a kind of fish)
'boy catching large' is the wordplay.
'boy' becomes 'benny' (**).
'catching' indicates putting letters inside.
'large' becomes 'l'.
'benny' placed around 'l' is 'BLENNY'.
(Other definitions for blenny that I've seen before include "slimy fish" , "Spiny-finned fish" .)