Worried about main threat? (1-4)
I believe the answer is:
u-boat
'main threat?' is the definition.
(main can mean open sea)
'worried about' is the wordplay.
'worried' is an anagram indicator.
'about' with letters rearranged gives 'U-BOAT'.
(Other definitions for u-boat that I've seen before include "German wartime submarine" , "German WW 2 sub" , "Take pool exercise" , "Submersible warship of WWII" , "Sea vessel" .)