Co-worker is friendly, though not at first (4)
I believe the answer is:
ally
'co-worker' is the definition.
'friendly though not at first' is the wordplay.
'friendly' becomes 'pally' (similar in meaning).
'though not at first' means to remove the first letter (the first letter is removed).
'pally' with its first letter taken off is 'ALLY'.
'is' acts as a link.
(Other definitions for ally that I've seen before include "Supporter in a conflict" , "Join by marriage, treaty etc" , "Cooperating country" , "Co-operating person or nation" , "often helpful when cluing adverbs" .)