An outright kill to sustain a hungry whale (5)
I believe the answer is:
krill
'whale' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for krill that I've seen before include "Tiny shrimps" , "Tiny crustaceans" , "Food of whalebone whales" , "Whale food" , "Crustaceans eaten by whales" .)