The Spanish soldier in plain clothes? (3)

I believe the answer is:

cid

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This clue could be a double definition.

'the spanish soldier' is the first definition.
The definition suggests a singular noun which matches the answer.

'plain clothes?' is the second definition.
The definition suggests a singular noun which matches the answer.

'in' acts as a link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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