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'Cm' and 'Ra': key when unmasking supplier of narcotics (8)

Ross

I believe the answer is:

mandrake

I'm a little stuck... Click here to teach me more about this clue!

'supplier of narcotics' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'cm and ra key when unmasking' is the wordplay.
'MANDRAKE' is hidden within the letters.
But, I am uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for mandrake that I've seen before include "Plant once believed to shriek when pulled up" , "Poisonous plant or cartoon magician" , "Herbal root" , "Eurasian plant with purplish flowers" , "Exotic plant" .)

I've seen this clue in The Telegraph.
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