Most exceptional artist on vacation (6)
I believe the answer is:
rarest
'most exceptional' is the definition.
(I've seen this in another clue)
'artist on vacation' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'on' means one lot of letters go next to another.
'vacation' becomes 'rest' (I've seen this in another clue).
'ra'+'rest'='RAREST'
(Other definitions for rarest that I've seen before include "Most scarce or least common" , "Sartre (anag) -- most uncommon" , "Most scarce and probably precious" , "fine to maximum" , "Most scarce or infrequent" .)