Sermonise (6)
I believe the answer is:
preach
'sermonise' is the definition.
(I know that sermonise is a more specific form of the action preach)
This is all the clue.
(Other definitions for preach that I've seen before include "Moralise" , "Proclaim (religion)" , "Give a religious address" , "A perch (anag.)" , "Give, eg, a sermon" .)