Course taken by some whales? (8)
I believe the answer is:
'course taken by some whales?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
This is the entire clue.
Can you help me to learn more?
(Other definitions for plankton that I've seen before include "Small organisms that float in water in great numbers" , "Microscopic organisms that float on the surface of water" , "food for school, possibly" , "Microscopic sea food" , "Whale's staple diet" .)