Cannon ball (6)
I believe the answer is:
pompom
This clue is a double definition.
'cannon' is the first definition.
(I've seen this before)
'ball' is the second definition.
(I've seen this before)
(Other definitions for pompom that I've seen before include "Woolly decoration" , "Decorative ball of tufted wool" , "Cheerleader's ball of material" , "Anti-aircraft cannon - woolly ball" , "Anti-aircraft weapon" .)