Explosive force catching duke on arm (9)
'explosive force' is the definition.
'duke on arm' is the wordplay.
'duke' becomes 'powder' (I can't explain this - if you can you should believe this answer much more).
'on' says to put letters next to each other.
'arm' becomes 'gun' (gun is a kind of arm).
'powder' put after 'gun' is 'GUNPOWDER'.
'catching' is the link.
(Other definitions for gunpowder that I've seen before include "plot? Sort of", "this plot failed", "Ingredient of fireworks", "likely to go up?", "Explosive mixture - plot of 1605".)