Opinion of boxer, perhaps Muhammad Ali originally (5)
I believe the answer is:
dogma
'opinion' is the definition.
(dogma means belief)
'boxer perhaps muhammad ali originally' is the wordplay.
'boxer perhaps' becomes 'dog' (boxer is a kind of dog).
'originally' says to take the initial letters.
The initial letters of 'muhammad ali' is 'ma'.
'dog'+'ma'='DOGMA'
'of' acts as a link.
(Other definitions for dogma that I've seen before include "Tenet" , "Body of creed" , "Fixed belief" , "Body of religious doctrine" , "Doctrine accepted as true without proof" .)