A weapon in playing billiards (6)
I believe the answer is:
Here is my best explanation:
This clue might be a double definition.
'a weapon' is the first definition.
(cannon is a kind of weapon)
'playing billiards' is the second definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'in' is the link.
(Other definitions for cannon that I've seen before include "One of the big guns", "Large gun that fires heavy shells", "Item of heavy artillery", "shot playing billiards", "Wheeled guns", "Large artillery piece", "Big shot", "Artillery gun", "Gun sounds like a churchman" and "Large gun mounted on wheels".)