The volume of students carried to class?
I believe the answer is:
schoolbook
'class?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'the volume of students carried' is the wordplay.
I cannot really see how this works, but
'volume' could be 'book' (book is a kind of volume) and 'book' is located in the answer.
'of' could be 'o' and 'o' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for schoolbook that I've seen before include "Teaching aid" , "Text" .)