Classes providing information on an epoch (6)
I believe the answer is:
genera
'classes' is the definition.
(I've seen this before)
'information on an epoch' is the wordplay.
'information' becomes 'gen' (gen is a kind of information).
'on' says to put letters next to each other.
'an epoch' becomes 'era' ('era' can be a synonym of 'epoch').
'gen'+'era'='GENERA'
'providing' is the link.
(Other definitions for genera that I've seen before include "similar groups" , "Group of organisms" , "Types" , "classes" , "Taxonomic groups" .)