Note wheels by motorway (5)
I believe the answer is:
minim
'note' is the definition.
(musical note)
'wheels by motorway' is the wordplay.
'wheels' becomes 'mini' (Mini is a type of car or 'wheels').
'by' means one lot of letters go next to another.
'motorway' becomes 'm'.
'mini'+'m'='MINIM'
(Other definitions for minim that I've seen before include "It's a downward stroke" , "Palindromic note" , "Feature of a score" , "Note equal to two crochets" , "Short note" .)