Crossword Genius

Convoy threat moving about (1-4)

Ross

I believe the answer is:

u-boat

I'm a little stuck... Click here to teach me more about this clue!

'convoy threat' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'moving about' is the wordplay.
'moving' is an anagram indicator.
'about' is an anagram of 'U-BOAT'.

Can you help me to learn more?

(Other definitions for u-boat that I've seen before include "About (anag)" , "Submersible warship armed with torpedoes" , "German submarine of WW2" , "German World War One or World War Two submarine" , "Take pool exercise" .)

I've seen this clue in The Sun.
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