Bail must be arranged in cabin for swimmer (7)
I believe the answer is:
Here is my best explanation:
'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 "Utah lib (anag)", "Marine food fish", "Large food fish", "Edible flatfish", "Fishy sort", "One's scaled", "Something fishy here", "Large flatfish used as food", "Flatfish" and "Fish found in Bali hut".)