Cop punching old dog, grass … (8)

I believe the answer is:

cannabis

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'grass' is the definition.
(I've seen this before)

'cop punching old dog' is the wordplay.
'cop' becomes 'nab' (synonyms).
'punching' indicates putting letters inside (I've seen this in other clues).
'old dog' becomes 'canis' (I've seen this before. I am not sure about the 'old' bit.).
'nab' going into 'canis' is 'CANNABIS'.

Can you help me to learn more?

(Other definitions for cannabis that I've seen before include "Grass" , "Hemp plant, illegal intoxicant drug" , "Ban Incas from using this drug" , "Marijuana" , "Soft drug" .)

I've seen this clue in The Guardian.

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