Progressive artist rejected LSD and left (7)
I believe the answer is:
radical
'progressive' is the definition.
(thesaurus)
'artist rejected lsd and left' is the wordplay.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'rejected' is a reversal indicator.
'lsd' becomes 'acid' ('acid' can be a synonym of 'lsd').
'and' says to put letters next to each other.
'left' becomes 'l' (common abbreviation).
'acid' written backwards gives 'dica'.
'ra'+'dica'+'l'='RADICAL'
(Other definitions for radical that I've seen before include "Holding extreme political views" , "Going to the root" , "Drastic and thorough, as reforms may be" , "Going to the roots, fundamental as change may be" , "Total" .)