German vehicle would be sound if it had a wheel (4)

I believe the answer is:

audi

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'german vehicle would be sound if it' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'a wheel' is the wordplay.
'wheel' becomes 'udi' (I can't explain this - if you can you should believe this answer much more).
'a'+'udi'='AUDI'

'had' acts as a link.

Can you help me to learn more?

(Another definition for audi that I've seen is " Car".)

I've seen this clue in The Independent.

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