Boxer maybe supported by mother’s belief (5)
I believe the answer is:
dogma
'belief' is the definition.
(dogma is a kind of belief)
'boxer maybe supported by mother's' is the wordplay.
'boxer maybe' becomes 'dog' (boxer is a breed of dog).
'supported by' says to put letters next to each other (letters go below - in down clue).
'mother' becomes 'ma' (Ma is another word for mother or mum).
'dog'+'ma'='DOGMA'
(Other definitions for dogma that I've seen before include "Non-evidence based opinion" , "Beliefs laid down as incontrovertibly true" , "Fixed belief" , "Doctrine accepted as true without proof" , "Body of creed" .)