Sub
'sub' is the definition.
(I know that U-boat is a type of attack submarine)
This is all the clue.
(Other definitions for u-boat that I've seen before include "German submarine of WW2" , "Take pool exercise" , "Submersible warship armed with torpedoes" , "German WW 2 sub" , "German wartime submarine" .)
'sub' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alternate that I've seen before include "In turns, first one then the other" , "Interchange" , "Happen in turns, rotate" , "Change repeatedly between two" , "Happening or following in turns, one after the other" .)
(Other definitions for step in that I've seen before include "Substitute" , "enter" , "Shoe with no fastenings" , "Type of garment" , "Intervene dramatically, or at a senior level" .)