Fish in barrel at front of aquarium (4)
I believe the answer is:
tuna
'fish' is the definition.
(tuna fish is a kind of fish)
'barrel at front of aquarium' is the wordplay.
'barrel' becomes 'tun' (tun is a kind of barrel**).
'at' means one lot of letters go next to another.
'front of' suggests taking the first letters.
The first letter of 'aquarium' is 'a'.
'tun'+'a'='TUNA'
'in' is the link.
(Other definitions for tuna that I've seen before include "A fish; sounds like piano juster" , "Bluefin, e.g." , "Large edible sea fish, often canned" , "Fish, often tinned" , "Popular food fish" .)