Some secret U-boat? (3)
I believe the answer is:
tub
'some secret u-boat?' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'some secret u-boat?' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'TUB' is hidden within 'secret u boat'.
Can you help me to learn more?
(Other definitions for tub that I've seen before include "Bath; clumsy ship" , "Kind of bath" , "An old-fashioned bath" , "An informal bath or boat" , "'Broad, open vessel (3)'" .)