Colleague is friendly, but not initially (4)
I believe the answer is:
ally
'colleague' is the definition.
(ally can mean a colleague or associate)
'friendly but not initially' is the wordplay.
'friendly' becomes 'pally' (similar in meaning).
'but not initially' means to remove the first letter.
'pally' with its initial letter taken away is 'ALLY'.
'is' is the link.
(Other definitions for ally that I've seen before include "Supporter in a conflict" , "State formally cooperating with another" , "He fights on your side" , "A friendly and reliable country" , "Join by marriage, treaty etc" .)