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' indicates an anagram (I've seen 'swirls' mean this).
'in the' means one lot of letters goes inside another.
'fish' becomes 'ling' (ling is a kind of fish).
'sea' with letters rearranged gives 'eas'.
'eas' going inside 'ling' is 'LEASING'.
'the' acts as a link.
(Other definitions for leasing that I've seen before include "Financial arrangement" , "Renting" , "Chartering" , "A single (anag.)" , "Letting for money" .)