Underwater menace swimming about (1-4)

I believe the answer is:

u-boat

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'underwater menace' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'swimming about' is the wordplay.
'swimming' is an anagram indicator.
'about' with letters rearranged gives 'U-BOAT'.

Can you help me to learn more?

(Other definitions for u-boat that I've seen before include "Submersible warship armed with torpedoes" , "Submersible warship of WWII" , "German submarine of WW2" , "Sea vessel" , "Take pool exercise" .)

I've seen this clue in The Sun and The Times.

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