It's most uncommon for artist to relax (6)
I believe the answer is:
rarest
'it's most uncommon' is the definition.
(I've seen this before)
'artist to relax' is the wordplay.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'to relax' becomes 'rest' (I've seen this before).
'ra'+'rest'='RAREST'
'for' is the link.
(Other definitions for rarest that I've seen before include "Most scarce or uncommon" , "most excellent" , "Most scarce and probably precious" , "Most scarce or least common" , "least done" .)