Umpteen barnacles on another tanker, first of all - vessel going under (1-4)

I believe the answer is:

u-boat

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'all vessel going under' is the definition.
I can't judge whether this definition defines the answer.

'umpteen barnacles on another tanker first' is the wordplay.
'first' suggests taking the first letters.
The first letters of 'umpteen barnacles on another tanker' is 'U-BOAT'.

'of' is the link.

Can you help me to learn more?

(Other definitions for u-boat that I've seen before include "Sea vessel" , "Submersible warship armed with torpedoes" , "Submersible warship of WWII" , "German wartime submarine" , "German WW 2 sub" .)

I've seen this clue in The Telegraph.

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