Swimmer and boxer born in shack (7)
I believe the answer is:
halibut
'swimmer' is the definition.
(halibut is a kind of aquatic vertebrate)
'boxer born in shack' is the wordplay.
'boxer' becomes 'ali' (Muhammad Ali).
'born' becomes 'b' (abbreviation next to the year of someone's birth).
'in' means one lot of letters goes inside another.
'shack' becomes 'hut' (synonyms).
'ali'+'b'='alib'
'alib' inserted inside 'hut' is 'HALIBUT'.
'and' is the link.
(Other definitions for halibut that I've seen before include "Swimmer" , "Largest flatfish" , "Something fishy here" , "Fishy sort" , "Fish found in Bali hut" .)