Preferably the artist on the right (6)
I believe the answer is:
Here is my best explanation:
'preferably' is the definition.
('rather' can be a synonym of 'preferably')
'the artist on the right' is the subsidiary indication.
'the artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'on' says to put letters next to each other.
'right' becomes 'r' (common abbreviation).
(Other definitions for rather that I've seen before include "More precisely; instead of", "On the contrary; sooner", "In preference", "Fairly", "somewhat > sooner? > Quite", "Pretty", "Slightly, more so than not (6", "Quite, sort of", "Sooner; somewhat" and "To a limited degree".)