Fourth man group hated at first (4)
I believe the answer is:
seth
'fourth man' is the definition.
(fourth man after Adam, Cain and Abel)
'group hated at first' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'at first' suggests taking the first letters.
The first letter of 'hated' is 'h'.
'set'+'h'='SETH'
(Other definitions for seth that I've seen before include "Boy" , "Indian writer" , "Adam's third son" , "Biblical character" , "said to sound archaic" .)