Bail must be arranged in cabin for swimmer (7)
I believe the answer is:
halibut
'swimmer' is the definition.
(halibut is a kind of aquatic vertebrate)
'bail must be arranged in cabin' is the wordplay.
'must be arranged' indicates an anagram.
'in' means one lot of letters goes inside another.
'cabin' becomes 'hut'.
'bail' is an anagram of 'alib'.
'alib' placed into 'hut' is 'HALIBUT'.
'for' is the link.
(Other definitions for halibut that I've seen before include "One's scaled" , "Large flatfish used as food" , "Utah lib (anag)" , "Largest flatfish" , "Fishy sort" .)