Fish barrel, say, without cover (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna is a type of fish)
'barrel say without cover' is the wordplay.
'barrel' becomes 'tun' (tun is a kind of barrel).
'without cover' means to look at the middle letters (the letters which cover up the word are removed).
The middle letter of 'say' is 'a'.
'tun'+'a'='TUNA'
(Other definitions for tuna that I've seen before include "Type of fish" , "(Meat of) large fish" , "Large fish for the strange aunt" , "Large tasty fish, fond of a nut" , "Yellow-finned fish" .)