Car’s old but sound (5)
I believe the answer is:
audio
'sound' is the definition.
(synonyms)
'car's old' is the wordplay.
'car' becomes 'audi' (audi is a german car brand).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'audi'+'o'='AUDIO'
'but' is the link.
(Other definitions for audio that I've seen before include "Denoting sound" , "- Tape; - typist" , "type of reproduction" , "Relating to sound or hearing" , "Electronic sound reproduction" .)