Agent going underground in Smolensk? (4)

I believe the answer is:

mole

I'm a little stuck... Click here to teach me more about this clue!

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

'going underground in smolensk?' is the wordplay.
'MOLE' is hidden within the letters.
However, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for mole that I've seen before include "Spy; burrower" , "Tunneller" , "Spy - one that burrows" , "Skin blemish and burrowing animal" , "A spy within an organisation" .)

I've seen this clue in the Evening Standard.

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