Detonator needs one good American gunpowder ingredient (7)
I believe the answer is:
igniter
'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').
'i'+'g'+'niter'='IGNITER'
'needs' is the link.
(Other definitions for igniter that I've seen before include "Lighter" , "Apparatus for firing an explosive" , "Explosion initiator" , "Combustion initiator" , "One sets off" .)