Drive initially identifying my favourite American? (7)
I believe the answer is:
impetus
'drive' is the definition.
(impetus is a kind of drive)
'initially identifying my favourite american?' is the wordplay.
'initially' suggests taking the first letters.
'favourite' becomes 'pet' ('pet' can be a synonym of 'favourite').
'american?' becomes 'us'.
The first letters of 'identifying my' is 'im'.
'im'+'pet'+'us'='IMPETUS'
(Other definitions for impetus that I've seen before include "influence" , "Imputes to force with which a body moves" , "Motivation" , "Thrust" , "Incentive - time's up (anag)" .)