Note car reversing on motorway (5)
I believe the answer is:
minim
'note' is the definition.
(type of musical note)
'car reversing on motorway' is the wordplay.
'car' becomes 'mini' (Mini Cooper).
'reversing' shows that the letters should be reversed in order.
'on' is a charade indicator (letters next to each other).
'motorway' becomes 'M' (as in motorway names - Mm, Mi etc).
'mini' back-to-front is 'inim'.
'inim' after 'm' is 'MINIM'.
(Other definitions for minim that I've seen before include "Two crotchets in one" , "Short note" , "Double crotchet" , "note or > dose" , "Half-semibreve" .)