Most unusual artist on break (6)
I believe the answer is:
rarest
'most unusual' is the definition.
(I've seen this before)
'artist on break' is the wordplay.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'on' says to put letters next to each other.
'break' becomes 'rest' (resting is a kind of breaking**).
'ra'+'rest'='RAREST'
(Other definitions for rarest that I've seen before include "Most scarce or uncommon" , "most excellent" , "fine to maximum" , "Most scarce or least common" , "Most scarce and probably precious" .)