After conversion, Preston idiot becomes a healer (12)
I believe the answer is:
periodontist
'a healer' is the definition.
'after conversion preston idiot' is the wordplay.
'after conversion' indicates anagramming the letters (I've seen this in other clues).
'preston'+'idiot'='prestonidiot'
'prestonidiot' anagrammed gives 'PERIODONTIST'.
'becomes' is the link.
(Other definitions for periodontist that I've seen before include "Gum expert" , "biting specialist" .)