Fool's derisive remark boxing good man (6)
I believe the answer is:
jester
'fool's' is the definition.
(synonyms)
'derisive remark boxing good man' is the wordplay.
'derisive remark' becomes 'jeer' (I've seen this in another clue).
'boxing' means one lot of letters goes inside another ('box' can mean contain, as in to put in a box).
'good man' becomes 'st' (abbreviation for saint).
'jeer' placed around 'st' is 'JESTER'.
(Other definitions for jester that I've seen before include "Court entertainer (with bells on?)" , "Court humorist" , "Court clown" , "Court fool" , "Professional joker" .)