Artist joins the others - most unusual (6)
I believe the answer is:
rarest
'most unusual' is the definition.
(I've seen this before)
'artist joins the others' is the wordplay.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'joins' is a charade indicator (letters next to each other).
'the others' becomes 'rest' (I've seen this in other clues).
'ra'+'rest'='RAREST'
(Other definitions for rarest that I've seen before include "Least common, most unusual" , "most excellent" , "Most scarce and probably precious" , "fine to maximum" , "least done" .)