Emerald, perhaps one with a hole in it? (5)

I believe the answer is:

green

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'emerald perhaps' is the definition.
(I know that emerald is a type of green)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for green that I've seen before include "Ecologically beneficial" , "The colour of envy" , "Not yet ripe" , "one of eighteen" , "member of political party" .)

I've seen this clue in The Independent.

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