Agree with stoic about classes (10)
I believe the answer is:
categories
'classes' is the definition.
('category' can be a synonym of 'class')
'agree with stoic about' is the wordplay.
'with' is a charade indicator (letters next to each other).
'about' is an anagram indicator.
'agree' put after 'stoic' is 'stoicagree'.
'stoicagree' with letters rearranged gives 'CATEGORIES'.
(Other definitions for categories that I've seen before include "classes" , "classifications" .)