Food for whales (5)
I believe the answer is:
krill
'food for whales' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for krill that I've seen before include "Whales' staple diet" , "Crustaceans, diet of whales" , "Crustaceans eaten by whales" , "What baleen whales eat" , "Small aquatic creatures" .)