Letting the sea swirl in the fish (7)
I believe the answer is:
leasing
'letting' is the definition.
('lease' can be a synonym of 'let')
'sea swirl in the fish' is the wordplay.
'swirl' is an anagram indicator (I've seen 'swirled' mean this).
'in the' indicates putting letters inside.
'fish' becomes 'ling' (ling is a kind of fish).
'sea' anagrammed gives 'eas'.
'eas' inserted into 'ling' is 'LEASING'.
'the' is the link.
(Other definitions for leasing that I've seen before include "Financial arrangement" , "Renting" , "Chartering" , "A single (anag.)" , "Letting for money" .)