Class, say, backed by student group (5)
I believe the answer is:
genus
'class' is the definition.
(thesaurus)
'say backed by student group' is the wordplay.
'say' becomes 'eg'.
'backed' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'student group' becomes 'nus' (National Union of Students).
'eg' backwards is 'ge'.
'ge'+'nus'='GENUS'
(Other definitions for genus that I've seen before include "The Homo of Homo sapiens?" , "Class of animal" , "Taxonomic grouping" , "Taxonomic category above species" , "Latin class?" .)