Bill goes under motorway alongside US vehicle (7)
I believe the answer is:
minibus
'vehicle' is the definition.
(I've seen this before)
'bill goes under motorway alongside us' is the wordplay.
'bill' becomes 'nib' (synonyms).
'goes under' means one lot of letters go next to another.
'motorway' becomes 'MI' (resembles Mn, English motorway).
'alongside' is a charade indicator (letters next to each other).
'nib' put after 'mi' is 'minib'.
'minib'+'us'='MINIBUS'
(Other definitions for minibus that I've seen before include "Small multi-seater vehicle" , "form of transport" , "Small coach" , "transport for children?" , "Vehicle for up to, say, 15 passengers" .)