A vessel (among the last U-boats?) (3)
I believe the answer is:
tub
'a vessel' is the definition.
(tub is a kind of vessel)
'among the last u-boats?' is the wordplay.
'among' indicates the answer is hidden within the clue.
'TUB' is hidden in the letters of 'last u boats'.
(Other definitions for tub that I've seen before include "Unseaworthy vessel" , "Large plant pot" , "Large open vessel" , "Kind of bath" , "Bath; clumsy ship" .)