... fish in a barrel to begin with (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna fish is a kind of fish)
'a barrel to begin with' is the wordplay.
'a barrel' becomes 'tun' (tun is a kind of barrel).
'to begin with' becomes 'a' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'tun'+'a'='TUNA'
'in' is the link.
Can you help me to learn more?
(Other definitions for tuna that I've seen before include "Large edible sea fish, often canned" , "Fish; edible cactus" , "Tunny" , "Tinned food fish" , "Bluefin, e.g." .)