Excellent artist, extremely reasonable (4)
I believe the answer is:
rare
'excellent' is the definition.
(I've seen this before)
'artist extremely reasonable' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'reasonable' with its middle removed is 're'.
'ra'+'re'='RARE'
(Other definitions for rare that I've seen before include "Singular, infrequent" , "not often spotted" , "Seldom seen or met" , "Meat that's lightly cooked" , "Ben Jonson was considered so" .)