Turnabout in the vessel (1-4)
I believe the answer is:
u-boat
This could be a double definition.
'turnabout' is the first definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'vessel' is the second definition.
(I know that U-boat is a type of submersible)
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for u-boat that I've seen before include "German wartime submarine" , "Take pool exercise" , "German World War One or World War Two submarine" , "About (anag)" , "German submarine of WW2" .)