Making impure drug, cutting quantity for a grown-up (12)
I believe the answer is:
adulteration
'making impure drug cutting' is the definition.
I can't tell whether this definition defines the answer.
'quantity for a grown-up' is the wordplay.
'quantity' becomes 'e' (e is a kind of quantity).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'a grown' becomes 'adult' (I've seen this before).
'up' becomes 'ration' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'e' put after 'adult' is 'adulte'.
'adulte'+'ration'='ADULTERATION'
Can you help me to learn more?
(Other definitions for adulteration that I've seen before include "Debasement" , "pollution" , "Corruption" .)