Road up on favourite American drive (7)
I believe the answer is:
impetus
'drive' is the definition.
(impetus is a kind of drive)
'road up on favourite american' is the wordplay.
'road' becomes 'mi' (**).
'up' is a reversal indicator (in down clue: letters go upwards).
'on' is a charade indicator (letters next to each other).
'favourite' becomes 'pet' ('pet' can be a synonym of 'favourite').
'american' becomes 'us'.
'mi' in reverse letter order is 'im'.
'im'+'pet'+'us'='IMPETUS'
(Other definitions for impetus that I've seen before include "Moving force" , "Catalyst" , "Driving force, forward movement" , "Motivation" , "Drive" .)