Quiet commercial for fish (4)
I believe the answer is:
shad
'for fish' is the definition.
(shad is a kind of fish)
'quiet commercial' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'commercial' becomes 'ad' (abbreviation for advertisement).
'sh'+'ad'='SHAD'
(Other definitions for shad that I've seen before include "Fish related to the herring" , "fish that's been caught" , "Herring-like fish" , "Fish; dash (anag.)" .)