Artist turning to a bitter, twisted judge (9)
I believe the answer is:
arbitrate
'judge' is the definition.
(to arbitrate is to judge or decide)
'artist turning to a bitter twisted' is the wordplay.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'turning' says the letters should be written in reverse.
'to' means one lot of letters go next to another.
'twisted' indicates anagramming the letters.
'ra' backwards is 'ar'.
'a'+'bitter'='abitter'
'abitter' is an anagram of 'bitrate'.
'ar'+'bitrate'='ARBITRATE'
(Other definitions for arbitrate that I've seen before include "Decide" , "Mediate" , "Help settle a dispute" , "judge" , "Adjudge" .)