One who fought J. Frazier with extremely combative hostility (6)
I believe the answer is:
malice
'hostility' is the definition.
(thesaurus)
'one who fought j frazier with extremely combative' is the wordplay.
'one who fought j frazier' becomes 'M Ali' (Muhammad Ali fought Joe Frazier).
'with' says to put letters next to each other.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'combative' with its centre removed is 'ce'.
'mali'+'ce'='MALICE'
(Other definitions for malice that I've seen before include "Desire to harm others, may be aforethought" , "Spite - desire to hurt" , "Bitterness" , "'Ill-will, spite (6)'" , "Hate" .)