Churchgoer reformed lag in prison (8)
I believe the answer is:
anglican
'churchgoer' is the definition.
(member of the Anglican church )
'reformed lag in prison' is the wordplay.
'reformed' is an anagram indicator.
'prison' becomes 'can' (the can is American slang for prison).
'lag'+'in'='lagin'
'lagin' is an anagram of 'angli'.
'angli'+'can'='ANGLICAN'
(Other definitions for anglican that I've seen before include "Member of the Church of England" , "Religious person" , "C of E person" , "As believer" , "Belonging to the Church of England" .)