Vessel about to be destroyed (1-4)
I believe the answer is:
u-boat
'vessel' is the definition.
(U-boat is an example)
'about to be destroyed' is the subsidiary indication.
'to be destroyed' is an anagram indicator.
'about' with letters rearranged gives 'uboat'.
(Other definitions for u-boat that I've seen before include "Submersible warship armed with torpedoes" , "German WW 2 sub" , "Take pool exercise" , "Sea vessel" , "Submersible warship of WWII" .)