Note car mileage initially (5)
I believe the answer is:
minim
'note' is the definition.
(type of musical note)
'car mileage initially' is the wordplay.
'car' becomes 'mini' (Mini Cooper).
'initially' suggests taking the first letters.
The first letter of 'mileage' is 'm'.
'mini'+'m'='MINIM'
(Other definitions for minim that I've seen before include "Half-note" , "Double crotchet" , "Two crotchets in one" , "Musical note - small amount of liquid" , "Note with value of half a semibreve" .)