Moderate sermon is heard (6)
I believe the answer is:
lessen
'moderate' is the definition.
(thesaurus)
'sermon is heard' is the wordplay.
'sermon' becomes 'lesson' (I've seen this before).
'is heard' indicates a 'sounds like' (homophone) clue.
'lesson' sounds like 'LESSEN'.
(Other definitions for lessen that I've seen before include "Make smaller or fewer" , "Make fewer in number" , "Lower" , "Contract" , "Reduce in number" .)