Shutting up with the salt water fish (7)
I believe the answer is:
sealing
'shutting up' is the definition.
'with the salt water fish' is the wordplay.
'with' is a charade indicator (letters next to each other).
'the salt water' becomes 'sea' (I've seen this before).
'fish' becomes 'ling' (ling is a kind of fish).
'sea'+'ling' is 'SEALING'.
(Other definitions for sealing that I've seen before include "Making tight" , "Ratifying" , "Securely closing" , "Making leakproof?" , "Giving authorisation to" .)