Watering down of grown-up portion, with drug introduced (12)
I believe the answer is:
adulteration
'watering down' is the definition.
(to water something down is to adulterate it)
'grown-up portion with drug introduced' is the wordplay.
'grown-up' becomes 'adult' (informal term for an adult).
'portion' becomes 'ration' (ration is a kind of portion).
'with' says to put letters next to each other.
'drug' becomes 'e' (E is another name for ecstasy).
'introduced' is an insertion indicator.
'adult'+'ration'='adultration'
'adultration' going around 'e' is 'ADULTERATION'.
'of' acts as a link.
(Other definitions for adulteration that I've seen before include "Corruption" , "Debasement" , "pollution" .)