Sinking vessel about to disintegrate (1-4)
I believe the answer is:
u-boat
'sinking vessel' is the definition.
(vessel which might sink others)
'about to disintegrate' is the wordplay.
'to disintegrate' indicates an anagram (I've seen 'disintegrated' mean this).
'about' with letters rearranged gives 'U-BOAT'.
(Other definitions for u-boat that I've seen before include "German WW 2 sub" , "Sea vessel" , "About (anag)" , "Take pool exercise" , "Submersible warship armed with torpedoes" .)