Gunpowder constituent's partly disconcerting on reflection (5)
I believe the answer is:
nitre
'gunpowder constituent's' is the definition.
(gunpowder contains nitre)
'partly disconcerting on reflection' is the wordplay.
'partly' says the answer is hidden in the clue (part of the clue is used).
'on reflection' says the letters should be written backwards.
'ertin' is hidden within 'disconcerting'.
'ertin' backwards is 'NITRE'.
(Other definitions for nitre that I've seen before include "Inter (anag.)" , "constituent of gunpowder" , "Also known as salt-petre" , "found in a slow match?" , "Inert kind of saltpetre" .)