Supporter, on friendly terms, heading off (4)
I believe the answer is:
ally
'supporter' is the definition.
(I've seen this before)
'on friendly terms heading off' is the wordplay.
'on friendly terms' becomes 'pally' (I've seen this before).
'heading off' means to remove the first letter.
'pally' with its initial letter taken away is 'ALLY'.
(Other definitions for ally that I've seen before include "mate" , "Collaborator, affiliate" , "Friend, partner" , "Team up" , "Friend in time of need" .)