Sinking vessel about to explode (1-4)
I believe the answer is:
u-boat
'sinking vessel' is the definition.
(vessel which might sink others)
'about to explode' is the wordplay.
'to explode' indicates anagramming the letters.
'about' anagrammed gives 'U-BOAT'.
(Other definitions for u-boat that I've seen before include "German World War One or World War Two submarine" , "About (anag)" , "Sea vessel" , "Submersible warship armed with torpedoes" , "Take pool exercise" .)