Sinking vessel, but buoyant at regular intervals (1-4)
I believe the answer is:
u-boat
'sinking vessel' is the definition.
(vessel which might sink others)
'but buoyant at regular intervals' is the wordplay.
'at regular intervals' indicates alternate letters.
'but'+'buoyant'='butbuoyant'
The alternating letters of 'butbuoyant' are 'U-BOAT'.
(Other definitions for u-boat that I've seen before include "Submersible warship armed with torpedoes" , "Sea vessel" , "German submarine of WW2" , "German WW 2 sub" , "About (anag)" .)