I am, darling, American - the driving force! (7)
I believe the answer is:
'the driving force' is the definition.
(impetus is a kind of driving force)
'i am darling american' is the wordplay.
'i am' becomes 'im'.
'darling' becomes 'pet' ('pet' can be a synonym of 'darling').
'american' becomes 'us'.
(Other definitions for impetus that I've seen before include "Driving force, momentum" , "Incentive, stimulus" , "Motivation" , "influence" , "Driving force, forward movement" .)