Limner turned wizard (6)
I believe the answer is:
merlin
'wizard' is the definition.
(I've seen this before)
'limner turned' is the wordplay.
'turned' is an anagram indicator.
'limner' with letters rearranged 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" .)