Explosive force catching duke on arm (9)
I believe the answer is:
gunpowder
'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" .)