Sides with boxer finally winning on points (6)
I believe the answer is:
aligns
'sides' is the definition.
(to side with someone is to align oneself with them)
'boxer finally winning on points' is the wordplay.
'boxer' becomes 'ali' (Muhammad Ali).
'finally' says to take the final letters.
The last letters of 'winning on points' are 'gns'.
'ali'+'gns'='ALIGNS'
'with' is the link.
(Other definitions for aligns that I've seen before include "Lines up" , "Arranges in signal" , "Joins forces" , "Makes (a policy) fit (with another)" , "Arranges, in rows say" .)