An artist to depend on, but not often (6)
I believe the answer is:
rarely
'not often' is the definition.
('rarely' is the opposite to 'often')
'an artist to depend on' is the wordplay.
'an artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'to depend on' becomes 'rely' (I've seen this before).
'ra'+'rely'='RARELY'
'but' is the link.
(Other definitions for rarely that I've seen before include "Not very often, seldom in fact" , "Very occasionally" , "Hardly ever, most infrequently" , "when hundred-to-one horse wins?" , "Not done or occurring very often" .)