Grown-up to speak furiously, having swallowed drug (impure stuff) (10)
I believe the answer is:
adulterant
'impure stuff' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'grown-up to speak furiously having swallowed drug' is the wordplay.
'grown-up' becomes 'adult' (informal term for an adult).
'to' says to put letters next to each other.
'speak furiously' becomes 'rant' (ranting is a kind of speaking. I am not sure about the 'furiously' bit.).
'having swallowed' is an insertion indicator.
'drug' becomes 'e' (E is another name for ecstasy).
'adult'+'rant'='adultrant'
'adultrant' placed around 'e' is 'ADULTERANT'.
Can you help me to learn more?
(Another definition for adulterant that I've seen is " Pollutant".)