Crossword Genius

Worried about main threat? (1-4)

Ross

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" .)

I've seen this clue in The Sun.
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