One cad reformed by a clergyman (6)
I believe the answer is:
deacon
'a clergyman' is the definition.
(deacon is a kind of clergyman)
'one cad reformed' is the wordplay.
'reformed' is an anagram indicator.
'one'+'cad'='onecad'
'onecad' is an anagram of 'DEACON'.
'by' acts as a link.
(Other definitions for deacon that I've seen before include "Priest's assistant" , "Clergyman of low rank" , "Church officer" , "A member of the lower orders of clergy" , "Churchman" .)