Gospel backfiring on church body (7)
I believe the answer is:
evangel
'gospel' is the definition.
(synonyms)
'backfiring on church body' is the wordplay.
'backfiring' says the letters should be written in reverse.
'on' becomes 'leg' (cricket term, fielding positions).
'church body' becomes 'nave' (I've seen this in other clues).
'leg'+'nave'='legnave'
'legnave' back-to-front is 'EVANGEL'.
(Other definitions for evangel that I've seen before include "Gospel of the New Testament" , "Name applied to any of the four main Gospels" , "moral doctrine" , "Matthew maybe" , "good news for Christians!" .)