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Tunnel is one of mine (7)

Ross

I believe the answer is:

synonym

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This could be a double definition.

'tunnel' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'one of mine' is the second definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'is' acts as a link.

Can you help me to learn more?

(Other definitions for synonym that I've seen before include "A word that can be interchanged with another" , "Substitute brought in for a term" , "Quick crossword clue" , "Word that can replace another in context" , "Word that can be exchanged for another" .)

I've seen this clue in The Telegraph.
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