Genuinely not about to join forces (4)
I believe the answer is:
ally
'join forces' is the definition.
(to ally with someone is to join forces with them)
'genuinely not about' is the wordplay.
'genuinely' becomes 'really' (I've seen this before).
'not' suggests deleting specific letters.
'about' becomes 're' ('re' means regarding or about).
'really' with 're' taken out is 'ALLY'.
'to' acts as a link.
(Other definitions for ally that I've seen before include "Friendly nation" , "Friend in time of need" , "Confederate; supporter" , "Join - friend" , "A friendly and reliable country" .)