Classes officer cut short (6)
I believe the answer is:
genera
'classes' is the definition.
(I've seen this before)
'officer cut short' is the wordplay.
'officer' becomes 'general' (general is a kind of officer).
'cut short' means to remove the last letter.
'general' with its final letter removed is 'GENERA'.
(Other definitions for genera that I've seen before include "similar groups" , "classes" , "Biological groupings" , "Types" , "Taxonomic categories (biology)" .)