Unknown eight in German boat (5)
I believe the answer is:
yacht
'boat' is the definition.
(yachting is a kind of boating)
'unknown eight in german' is the wordplay.
'unknown' becomes 'y' (algebra).
'eight in german' becomes 'acht' (German translation of 'eight').
'y'+'acht'='YACHT'
(Other definitions for yacht that I've seen before include "Light sailing-vessel for sport" , "Luxury purchase" , "Sailing-boat" , "Sailing boat" , "Sailboat" .)