Supporter, one coming in following split (6)
I believe the answer is:
friend
'supporter' is the definition.
(synonyms)
'one coming in following split' is the wordplay.
'one' becomes 'i' (Roman numeral).
'coming in' is an insertion indicator.
'following' becomes 'f' (used when citing page numbers etc).
'split' becomes 'rend' (rend can mean to split or tear).
'f'+'rend'='frend'
'i' going inside 'frend' is 'FRIEND'.
(Other definitions for friend that I've seen before include "Buddy, mate" , "Person regarded with trust and affection" , "Close or intimate acquaintance" , "Pal, mate" , "allowed to pass by the guard" .)