Boxer dog is lit badly (8)
I believe the answer is:
pugilist
'boxer' is the definition.
(synonyms)
'dog is lit badly' is the wordplay.
'dog' becomes 'pug' (pug is a kind of dog).
'badly' indicates an anagram (letters in the wrong order).
'is'+'lit'='islit'
'islit' is an anagram of 'ilist'.
'pug'+'ilist'='PUGILIST'
(Other definitions for pugilist that I've seen before include "A professional boxer" , "Fist fighter" , "Prize fighter" .)