Artist, getting on, is seldom seen (4)
I believe the answer is:
rare
'seldom seen' is the definition.
(thesaurus)
'artist getting on' is the wordplay.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'getting' becomes 're' (I've seen this in other clues).
'on' says to put letters next to each other.
'ra'+'re'='RARE'
'is' is the link.
(Other definitions for rare that I've seen before include "Exceptionable" , "Excellent; uncommon" , "Seen once in a blue moon" , "like gold dust" , "Very, very occasional" .)