Secret agent necessarily underground? (4)

I believe the answer is:

mole

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'secret agent' is the definition.
(mole is a kind of secret agent)

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 "A spy within an organisation" , "Heap of soil (a 6 mountain?)" , "Small congenital skin spot" , "beauty spot" , "Naevus - burrowing mammal - spy" .)

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

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