Submarine about to be destroyed (1-4)
I believe the answer is:
u-boat
'submarine' is the definition.
('U-boat' can be a synonym of 'submarine')
'about to be destroyed' is the subsidiary indication.
'to be destroyed' indicates an anagram.
'about' with letters rearranged gives 'uboat'.
(Other definitions for u-boat that I've seen before include "Take pool exercise" , "Submersible warship armed with torpedoes" , "German submarine of WW2" , "Sea vessel" , "About (anag)" .)