Detonator needs one good American gunpowder ingredient (7)
I believe the answer is:
Here is my best explanation:
'detonator' is the definition.
'one good american gunpowder ingredient' is the subsidiary indication.
'one' becomes 'i' (roman numeral).
'good' becomes 'g' (abbreviation).
'american gunpowder ingredient' becomes 'niter' (US spelling of 'nitre').
'needs' is the link.
(Other definitions for igniter that I've seen before include "Combustion initiator", "It sets off explosive", "Device for starting combustion", "Device for lighting fuel in an engine", "Apparatus for firing an explosive", "Combustion starter", "Device for detonating fuel in an engine" and "Explosion initiator".)