Boxer dog - one by heel (8)
I believe the answer is:
pugilist
'boxer' is the definition.
('pugilist' can be a synonym of 'boxer')
'dog one by heel' is the wordplay.
'dog' becomes 'pug' (pug is a kind of dog).
'one' becomes 'i' (Roman numeral).
'by' means one lot of letters go next to another.
'heel' becomes 'list' (I've seen this in other clues).
'pug'+'i'+'list'='PUGILIST'
(Other definitions for pugilist that I've seen before include "Fist fighter" , "A professional boxer" , "Prize fighter" .)