Powder used in metal cannon (4)
I believe the answer is:
talc
'powder' is the definition.
(I know that talc is a more specific form of the action powder)
'in metal cannon' is the wordplay.
'in' says the answer is hidden in the clue.
'TALC' can be found hidden inside 'metal cannon'.
'used' is the link.
(Other definitions for talc that I've seen before include "Perfumed drying powder" , "White powder for the bathroom" , "Drier" , "soother" , "Soft stuff" .)