German car wheel is sound (5)
I believe the answer is:
audio
'is sound' is the definition.
(audio means sound)
'german car wheel' is the wordplay.
'german car' becomes 'audi' (I've seen this in another clue).
'wheel' becomes 'O' (O looks like a wheel).
'audi'+'o'='AUDIO'
(Other definitions for audio that I've seen before include "Electronic sound reproduction" , "Relating to sound or hearing" , "TV sound" , "- Tape; - typist" , "Prefix indicating hearing or sound" .)