Large cask on top of a fish (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna is a type of fish)
'large cask on top of a' is the wordplay.
'large cask' becomes 'tun' (type of container for alcohol).
'on top of' means one lot of letters go next to another.
'tun'+'a'='TUNA'
(Other definitions for tuna that I've seen before include "Large salt water fish" , "Popular food fish" , "Large edible sea fish, often canned" , "Widely-eaten fish" , "Large food fish" .)