The majority of officer classes? (6)
I believe the answer is:
genera
'classes?' is the definition.
(more than one genus - a class or category)
'the majority of officer' is the wordplay.
'the majority of' means to remove the last letter (most but not all of the word is used).
'officer' becomes 'general' (general is a kind of officer).
'general' with its last letter taken off is 'GENERA'.
(Other definitions for genera that I've seen before include "Orders with common characteristics" , "Group of organisms" , "Groups with common characteristics" , "Sorts" , "Forms" .)