Meeting of clergy into heresy, no doubt (5)
I believe the answer is:
synod
'meeting of clergy' is the definition.
(a synod is an assembly of the clergy)
'into heresy no doubt' is the wordplay.
'into' says the answer is hidden in the clue.
'SYNOD' can be found hidden inside 'heresy no doubt'.
(Other definitions for synod that I've seen before include "Ecumenical council" , "Assembly of the clergy" , "Forum for discussing" , "group of clergymen" , "council meeting" .)