This will undermine the breakwater (4)

I believe the answer is:

mole

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'the breakwater' is the definition.
('mole' can be a synonym of 'breakwater')

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for mole that I've seen before include "one in the Underworld?" , "Spy, skin spot and furry animal" , "Counterspy" , "Groyne" , "spy a > breakwater" .)

I've seen this clue in The Irish News.

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