I'm amused by fat religious reformer (7)
I believe the answer is:
lollard
'religious reformer' is the definition.
(Lollards were a pre-Reformation Christian movement)
'i'm amused by fat' is the wordplay.
'i'm amused' becomes 'lol' (abbreviation for laughing out loud).
'by' is a charade indicator (letters next to each other).
'fat' becomes 'lard' (lard is a kind of fat).
'lol'+'lard'='LOLLARD'
(Other definitions for lollard that I've seen before include "religious reformer" , "Follower of John Wycliffe" , "Follower of John Wyclif" , "Wycliffite" , "Radical Christian" .)