Shells, perhaps, in downstream motion (4)
I believe the answer is:
ammo
'shells perhaps' is the definition.
(I know that cartridge is a type of ammo)
'in downstream motion' is the wordplay.
'in' says the answer is hidden in the clue.
'AMMO' is hidden in the letters of 'downstream motion'.
(Other definitions for ammo that I've seen before include "Explosive devices" , "Bullets, shells, rockets etc" , "Slugs perhaps" , "Bullets, shells, etc. (abbr.)" , "Contents of magazine" .)