Agree to start off as a friend (4)
I believe the answer is:
ally
'a friend' is the definition.
('ally' can be a synonym of 'friend')
'agree to start off' is the wordplay.
'agree' becomes 'tally' ('tally' can be a synonym of 'agree').
'to start off' means to remove the first letter (I've seen 'starting off' mean this (the starting letter is removed)).
'tally' with its first letter taken off is 'ALLY'.
'as' is the link.
(Other definitions for ally that I've seen before include "mate" , "Co-operative friend" , "Join by marriage, treaty etc" , "Join forces with" , "Fighting partner" .)