Two informants – one going underground (4,3)

I believe the answer is:

mole rat

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'one going underground' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'two informants' is the wordplay.
'two informants' means two replacements for 'informant'.
'informant' becomes 'mole'.
'informant' becomes 'rat' (rat is a kind of informant).
'mole'+'rat'='MOLE-RAT'

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I've seen this clue in The Independent.

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