Unknown number in German boat (5)

I believe the answer is:

yacht

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'boat' is the definition.
(yacht is a type of boat)

'unknown number in german' is the wordplay.
'unknown number' becomes 'y' (I've seen this in other clues).
'in german' becomes 'acht' (I can't explain this - if you can you should believe this answer much more).
'y'+'acht'='YACHT'

Can you help me to learn more?

(Other definitions for yacht that I've seen before include "Sailing boat" , "Pleasure craft" , "Sailing craft" , "Light, fast sailing-vessel" , "Light sailing-vessel for sport" .)

I've seen this clue in The Independent.

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