Fishtail seen on a badly built fish (7)
I believe the answer is:
halibut
'fish' is the definition.
(halibut is a kind of fish)
'fishtail seen on a badly built' is the wordplay.
'fishtail' becomes 'ha' (I can't explain this - if you can you should give a lot more credence to this answer).
'seen on' means one lot of letters go next to another.
'a badly' indicates an anagram.
'built' anagrammed gives 'libut'.
'ha'+'libut'='HALIBUT'
(Other definitions for halibut that I've seen before include "Large food fish" , "Marine food fish" , "Utah lib (anag)" , "Fish found in Bali hut" , "Large flatfish, tasty to eat" .)