Submarine (1-4)
'submarine' is the definition.
(submarine means under the sea)
This is the entire clue.
(Other definitions for undersea that I've seen before include "Below marine surface" , "Beneath the oceans" , "submerged" , "Submarine" , "Where bed is" .)
'submarine' is the definition.
(I know that submarine can be written as U-boat)
This is all the clue.
(Other definitions for u-boat that I've seen before include "Take pool exercise" , "German World War One or World War Two submarine" , "Sea vessel" , "Submersible warship of WWII" , "German WW 2 sub" .)