Crossword Genius

Minister in speech — one firing off? (6)

Ross

I believe the answer is:

cannon

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'one firing off?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'minister in speech' is the wordplay.
'minister' becomes 'canon' (canon is a type of church minister).
'in speech' indicates a 'sounds like' (homophone) clue.
'canon' sounds like 'CANNON'.

Can you help me to learn more?

(Other definitions for cannon that I've seen before include "Billiard shot or large gun" , "mortar" , "The large gun sounds like a church dignitary" , "Billiard shot in which player's ball hits the two others" , "military weapon" .)

I've seen this clue in The Guardian.
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