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 wordplay.
'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 "Lighter", "It may cause a flare-up", "Combustion starter", "Combustion initiator", "One sets off", "Device for detonating fuel in an engine", "It sets off explosive", "Device for lighting fuel in an engine", "Device for starting combustion" and "Apparatus for firing an explosive".)