Car with old sound system (5)
I believe the answer is:
audio
'sound system' is the definition.
(I've seen this before)
'car with old' is the wordplay.
'car' becomes 'audi' (audi is a german car brand).
'with' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'audi'+'o'='AUDIO'
(Other definitions for audio that I've seen before include "Television sound" , "Prefix indicating hearing or sound" , "Prefix signifying sound or hearing" , "Sound recording" , "Relating to sound or hearing" .)