He put two and two together and didnt go under (4)

I believe the answer is:

noah

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'he put two and two together' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for noah that I've seen before include "Oh, an ark man" , "Historic sailor" , "Sailor who ran aground on Mount Ararat" , "Biblical sailor" , "A very early sailor" .)

I've seen this clue in The Irish Independent.

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