Commit to flames popular heretic for speaking out — consumed! (10)
I believe the answer is:
incinerate
'commit to flames' is the definition.
(I've seen this in another clue)
'popular heretic for speaking out consumed' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'heretic' becomes 'sinner' (heretics are sinners).
'for speaking out' indicates a 'sounds like' (homophone) clue.
'consumed' becomes 'ate' (to eat is to consume food).
'sinner' is a homophone of 'ciner'.
'in'+'ciner'+'ate'='INCINERATE'
(Other definitions for incinerate that I've seen before include "Completely burn" , "Reduce to ash" , "Destroy by burning" , "Fierily destroy" , "Reduce to ashes" .)