Vessel to turn about (1-4)
I believe the answer is:
u-boat
'vessel' is the definition.
(I've seen this before)
'turn about' is the wordplay.
'turn' indicates an anagram.
'about' anagrammed gives 'U-BOAT'.
'to' is the link.
(Other definitions for u-boat that I've seen before include "Submersible warship of WWII" , "Sea vessel" , "German wartime submarine" , "German World War One or World War Two submarine" , "German WW 2 sub" .)