Whale food (5)
I believe the answer is:
'whale food' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for krill that I've seen before include "Very small crustaceans" , "Crustaceans eaten by whales" , "Small shrimp-like creatures eaten by whales" , "Tiny shrimps" , "Whale food" .)