Complete artist left bitter upset (7)
I believe the answer is:
radical
'complete' is the definition.
(thesaurus)
'artist left bitter upset' is the wordplay.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'left' becomes 'l' (common abbreviation).
'bitter' becomes 'acid' ('acid' can be a synonym of 'bitter'**).
'upset' shows that the letters should be reversed in order.
'l'+'acid'='lacid'
'lacid' backwards is 'dical'.
'ra'+'dical'='RADICAL'
(Other definitions for radical that I've seen before include "left-winger" , "Drastic or thorough as changes may be" , "stem" , "Person with extreme political views" , "Thoroughgoing - (popularly) extremist" .)