I'm with favourite American force (7)
I believe the answer is:
impetus
'force' is the definition.
(I know that impetus is a type of force)
'i'm with favourite american' is the wordplay.
'with' is a charade indicator (letters next to each other).
'favourite' becomes 'pet' (synonyms).
'american' becomes 'US' (United States).
'im'+'pet'+'us'='IMPETUS'
(Other definitions for impetus that I've seen before include "Driving force, momentum" , "Incentive, stimulus" , "Driving force, forward movement" , "Imputes to force with which a body moves" , "Incentive - time's up (anag)" .)