Clerical gentleman’s weapon in speech (5)
I believe the answer is:
canon
'clerical gentleman's' is the definition.
'weapon in speech' is the wordplay.
'weapon' becomes 'cannon' (cannon is a kind of weapon).
'in speech' shows a homophone (sound like).
'cannon' sounds like 'CANON'.
(Other definitions for canon that I've seen before include "Standard" , "Collection of writing" , "priest for cathedral duties" , "in the ministry" , "Big type" .)