Call in very quietly for shellfish (7)
I believe the answer is:
scallop
'shellfish' is the definition.
(scallop is a kind of shellfish)
'call in very quietly' is the wordplay.
'in' means one lot of letters goes inside another.
'very' becomes 'so' (both can mean 'extremely').
'quietly' becomes 'p' (p for piano and quiet).
'call' going into 'so' is 'scallo'.
'scallo'+'p'='SCALLOP'
'for' is the link.
(Other definitions for scallop that I've seen before include "Shellfish - pilgrim badge" , "Seafood - shell" , "Bivalve mollusc with fan-shaped shell" , "Marine mollusc" , "Edible bivalve mollusc" .)