Classes where dope given time (6)
I believe the answer is:
genera
'classes' is the definition.
(more than one genus - a class or category)
'dope given time' is the wordplay.
'dope' becomes 'gen' (both can mean information).
'given' means one lot of letters go next to another.
'time' becomes 'era' (an era is a period of time).
'gen'+'era'='GENERA'
'where' acts as a link.
(Other definitions for genera that I've seen before include "Types" , "Biological groupings" , "Groups between family and species (biology)" , "Groups with common characteristics" , "classes" .)