Progressive artist admitted to defamation (7)
I believe the answer is:
liberal
'progressive' is the definition.
(I know that progressive can be written as liberal)
'artist admitted to defamation' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'admitted to' is an insertion indicator.
'defamation' becomes 'libel' (libel is a kind of defamation).
'ra' inserted into 'libel' is 'LIBERAL'.
(Other definitions for liberal that I've seen before include "Bountiful" , "Ample; former politician" , "Generous, like a British political party" , "Braille (anag.)" , "Large-minded" .)