Others holding artist up? Most unusual (6)
I believe the answer is:
rarest
'most unusual' is the definition.
(I've seen this before)
'others holding artist up?' is the wordplay.
'others' becomes 'rest' (I've seen this in other clues).
'holding' is an insertion indicator.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'up?' is a reversal indicator (in down clue: letters go upwards).
'ra' written backwards gives 'ar'.
'rest' placed around 'ar' is 'RAREST'.
(Other definitions for rarest that I've seen before include "Least common, most unusual" , "least done" , "most excellent" , "Most scarce or least common" , "Most scarce or infrequent" .)