Quietly warn about seafood (5)
I believe the answer is:
prawn
'seafood' is the definition.
(I know that prawn is a type of seafood)
'quietly warn about' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'about' indicates anagramming the letters.
'warn' is an anagram of 'rawn'.
'p'+'rawn'='PRAWN'
(Other definitions for prawn that I've seen before include "basis for cocktail" , "Shrimp-like crustacean" , "Popular oriental snack" , "Shellfish" , "Large shrimp" .)