Whodunit? Investigate leads from the clues (5)

I believe the answer is:

green

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'whodunit?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I cannot 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 "Money" , "Environmentally-friendly" , "Ecologically correct" , "political activist" , "Ecologically sound" .)

I've seen this clue in The Independent.

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