Boxer born in shack gets fish (7)
I believe the answer is:
halibut
'fish' is the definition.
(halibut is a kind of fish)
'boxer born in shack' is the wordplay.
'boxer' becomes 'ali' (Muhammad Ali).
'born' becomes 'b'.
'in' is an insertion indicator.
'shack' becomes 'hut' (synonyms).
'ali'+'b'='alib'
'alib' placed inside 'hut' is 'HALIBUT'.
'gets' acts as a link.
(Other definitions for halibut that I've seen before include "Utah lib (anag)" , "Large edible flatfish" , "Large food fish" , "Fishy sort" , "Something fishy here" .)