Large cask has one fish (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna is a type of fish)
'large cask has one' is the wordplay.
'large cask' becomes 'tun' (type of container for alcohol).
'has' says to put letters next to each other.
'one' becomes 'a' (a thing is one thing).
'tun'+'a'='TUNA'
(Other definitions for tuna that I've seen before include "Yellow-finned fish" , "Type of fish" , "Large fish found in a nut" , "Common food fish" , "Warm-water food fish" .)