Swimmer having a party in shack (7)
I believe the answer is:
halibut
'swimmer' is the definition.
(halibut is a kind of aquatic vertebrate)
'a party in shack' is the wordplay.
'party' becomes 'lib' (abbreviation for Liberal, uk political party).
'in' means one lot of letters goes inside another.
'shack' becomes 'hut' ('hut' can be a synonym of 'shack').
'a'+'lib'='alib'
'alib' inserted inside 'hut' is 'HALIBUT'.
'having' acts as a link.
(Other definitions for halibut that I've seen before include "Fishy sort" , "Swimmer" , "Marine food fish" , "Fish found in Bali hut" , "Large flatfish, tasty to eat" .)