Minister is big gun, from what we hear (6)
I believe the answer is:
cannon
'minister' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I can't explain the remainder of the clue.
(Other definitions for cannon that I've seen before include "Large gun mounted on wheels" , "One of the big guns" , "Weapon firing large balls" , "Heavy ordnance" , "Billiard shot in which player's ball hits the two others" .)