Chemical evidence of cannabis smoking? (6)
I believe the answer is:
potash
This clue might be a double definition.
'chemical' is the first definition.
(I've seen this before)
'evidence of cannabis smoking?' is the second definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
Can you help me to learn more?
(Other definitions for potash that I've seen before include "Potassium compound used as fertiliser" , "Potassium carbonate or hydroxide" , "chemical compound" , "Alkaline compound" , "Strong alkali derived from burnt wood" .)