Spy going underground (4)

I believe the answer is:

mole

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This might be a double definition.

'spy' is the first definition.
(I know that mole is a type of spy)

'going underground' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?

Can you help me to learn more?

(Other definitions for mole that I've seen before include "Spot - traitor" , "Spy; blemish" , "Breakwater; naevus" , "Groyne" , "one in the Underworld?" .)

I've seen this clue in the Evening Standard.

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