Little Benjamin holds large fish (6)
I believe the answer is:
blenny
'fish' is the definition.
(blenny is a kind of fish)
'little benjamin holds large' is the wordplay.
'little benjamin' becomes 'benny' (I can't justify this - if you can you should give a lot more credence to this answer).
'holds' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'benny' going around 'l' is 'BLENNY'.
Can you help me to learn more?
(Other definitions for blenny that I've seen before include "Spiny-finned fish" , "slimy fish" .)