In action it resembles saltpetre (5)
I believe the answer is:
Here is my best explanation:
'saltpetre' is the definition.
('nitre' can be a synonym of 'saltpetre')
'in action it resembles' is the subsidiary indication.
'in' indicates the answer is hidden within the clue.
'nitre' can be found hidden inside 'action it resembles'.
(Other definitions for nitre that I've seen before include "Sodium nitrate", "Inert kind of saltpetre", "Gunpowder ingredient", "Mineral used in gunpowder", "Otherwise saltpetre", "Saltpetre", "Inert (anag) - saltpetre", "Also known as salt-petre" and "Inter (anag.)".)