Boxer embraced by Coot in unconventional union (9)
I believe the answer is:
coalition
'union' is the definition.
(coalition is a kind of union)
'boxer embraced by coot in unconventional' is the wordplay.
'boxer' becomes 'Ali' (Muhammad Ali).
'embraced by' means one lot of letters goes inside another.
'unconventional' indicates anagramming the letters.
'coot'+'in'='cootin'
'cootin' anagrammed gives 'cotion'.
'ali' put within 'cotion' is 'COALITION'.
(Other definitions for coalition that I've seen before include "Alliance of parties" , "form of government" , "Temporary alliance" .)