Vessel about to break up (1-4)
I believe the answer is:
u-boat
'vessel' is the definition.
(U-boat is an example)
'about to break up' is the subsidiary indication.
'to break up' indicates an anagram.
'about' anagrammed gives 'uboat'.
(Other definitions for u-boat that I've seen before include "German submarine of WW2" , "Take pool exercise" , "Submersible warship armed with torpedoes" , "German wartime submarine" , "German World War One or World War Two submarine" .)