Groups retrospectively brought in by fare negotiators (6)
I believe the answer is:
genera
'groups retrospectively' is the definition.
'genera' can be an answer for 'groups' (more than one biological genus). I am not sure about the 'retrospectively' bit.
'brought in by fare negotiators' is the wordplay.
'GENERA' is hidden backwards within the letters.
However, I'm uncertain how this is indicated.
Can you help me to learn more?
(Other definitions for genera that I've seen before include "Taxonomic categories (biology)" , "Sorts" , "Categories making up a family" , "classes" , "Groups between family and species (biology)" .)