Vessel unknown, number spoken by U-boat crew? (5)
I believe the answer is:
yacht
'vessel' is the definition.
(yacht is a kind of vessel)
'unknown number spoken by u-boat crew?' is the wordplay.
'unknown' becomes 'y' (algebra).
'number spoken by u-boat crew?' becomes 'acht' ('eight' in German - U-boats were German submarines).
'y'+'acht'='YACHT'
(Other definitions for yacht that I've seen before include "Cathy (anag.)" , "Pleasure-boat" , "Pleasure craft" , "Sailboat" , "Type of boat" .)