Limner turned wizard (6)
I believe the answer is:
merlin
'wizard' is the definition.
(I know that Merlin has the occupation the wizard)
'limner turned' is the wordplay.
'turned' is an anagram indicator.
'limner' anagrammed gives 'MERLIN'.
(Other definitions for merlin that I've seen before include "Bird of prey - magician" , "BBC series" , "could be relied on for a spell?" , "Kind of hawk or Camelot's magician" , "Arthurian wizard" .)