Fast U-boat bags slow ship (3)
I believe the answer is:
tub
'slow ship' is the definition.
'fast u-boat bags' is the wordplay.
'bags' says the answer is hidden in the clue.
'TUB' is hidden in the letters of 'fast u boat'.
(Other definitions for tub that I've seen before include "'Broad, open vessel (3)'" , "Maybe it sailed" , "Wide container; boat" , "Pot (of e.g. ice-cream)" , "Bath; leaky ship" .)