Where the pupils learn of whales (6)
I believe the answer is:
school
'where the pupils learn of whales' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
This is all the clue.
Can you help me to learn more?
(Other definitions for school that I've seen before include "Centre of learning" , "Academy; group of fish" , "Group of porpoises - place of education" , "Group of sea creatures" , "See 24" .)