German wartime submarine (5)
(Other definitions for uboat that I've seen before include "Warship" , "Vessel" , "What convoy tried to avoid" , "Sub" .)
'german wartime submarine' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for u-boat that I've seen before include "Submersible warship armed with torpedoes" , "About (anag)" , "Submersible warship of WWII" , "German wartime submarine" , "German WW 2 sub" .)