Dog — one lean boxer (8)
I believe the answer is:
pugilist
'boxer' is the definition.
(synonyms)
'dog one lean' is the wordplay.
'dog' becomes 'pug' (pug is a kind of dog).
'one' becomes 'I' (Roman numeral).
'lean' becomes 'list' (both can mean to tilt).
'pug'+'i'+'list'='PUGILIST'
(Other definitions for pugilist that I've seen before include "A professional boxer" , "Fist fighter" , "Prize fighter" .)