Evangelist on behalf of heathen is breaking limits of deceit (12)
I believe the answer is:
propagandist
'evangelist' is the definition.
(I've seen this before)
'on behalf of heathen is breaking limits of deceit' is the wordplay.
'on behalf of' becomes 'pro'.
'heathen' becomes 'pagan' (both can mean an irreligious person).
'breaking' is an insertion indicator.
'limits of' suggests removing the centre.
'deceit' with its middle removed is 'dt'.
'is' going into 'dt' is 'dist'.
'pro'+'pagan'+'dist'='PROPAGANDIST'
(Another definition for propagandist that I've seen is " evangelist".)