U-boat, for one (3)
I believe the answer is:
sub
'u-boat for one' is the definition.
(I know that U-boat is a type of attack submarine)
This is all the clue.
(Other definitions for sub that I've seen before include "Briefly, membership fee or stand-in" , "Prefix meaning beneath" , "An advance against income" , "Underwater craft" , "Beneath on bus" .)