Grimace about denial set down in sermon, perhaps (9)
I believe the answer is:
monologue
'sermon perhaps' is the definition.
(speech for one person)
'grimace about denial set down in' is the wordplay.
'grimace' becomes 'moue' (moue is a kind of grimace).
'about' indicates putting letters inside.
'denial' becomes 'no' (no is a kind of denial).
'set down in' becomes 'log' (I am not sure about this - if you are sure you should believe this answer much more).
'no'+'log'='nolog'
'moue' enclosing 'nolog' is 'MONOLOGUE'.
Can you help me to learn more?
(Other definitions for monologue that I've seen before include "Long solo speech" , "Long speech by one actor" , "Long and tedious speech" , "Long dramatic speech by a single actor" , "Long uninterrupted speech" .)