Supporter's friendly but not initially (4)
I believe the answer is:
ally
'supporter's' is the definition.
(I've seen this before)
'friendly but not initially' is the wordplay.
'friendly' becomes 'pally' ('pally' can be similar in meaning to 'friendly').
'but not initially' means to remove the first letter.
'pally' with its first letter taken off is 'ALLY'.
(Other definitions for ally that I've seen before include "He fights on your side" , "Collaborator, affiliate" , "Join - friend" , "Associate that provides assistance" , "Team up" .)