Important food fish (7)
(Other definitions for sardine that I've seen before include "(Cramped?) fish" , "Rinsed a (anag) - fish" , "Fish (often tinned)" , "FRENCH FISH" , "Young pilchard" .)
'important food fish' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for herring that I've seen before include "Small silvery sea-fish" , "Common food fish" , "one could be soused" , "Fish that can be red and kippered!" , "Fish, often kippered" .)