Artist tries going mad about one collector's items (8)
I believe the answer is:
rarities
'collector's items' is the definition.
(I've seen this in another clue)
'artist tries going mad about one' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'going mad' indicates anagramming the letters.
'about' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'tries' with letters rearranged gives 'rites'.
'rites' enclosing 'i' is 'rities'.
'ra'+'rities'='RARITIES'
(Other definitions for rarities that I've seen before include "Scarce and valuable things" , "Things seldom encountered" , "They don't happen often" , "Very unusual things" , "Uncommon things" .)