A saint corrupted, and saint becoming sinner? (8)
I believe the answer is:
satanist
'sinner?' is the definition.
'a saint corrupted and saint' is the wordplay.
'corrupted' is an anagram indicator.
'and' means one lot of letters go next to another.
'saint' becomes 'st'.
'a'+'saint'='asaint'
'asaint' is an anagram of 'satani'.
'satani'+'st'='SATANIST'
'becoming' is the link.
(Other definitions for satanist that I've seen before include "Devil devotee" , "Saint sat with devil worshipper" , "One who worships the devil" , "Devil-worshipper" , "diabolical fellow" .)