Corruption of grown-up portion, with Ecstasy in it (12)
I believe the answer is:
adulteration
'corruption' is the definition.
(I've seen this before)
'grown-up portion with ecstasy in it' is the wordplay.
'grown up' becomes 'adult' (I've seen this before).
'portion' becomes 'ration' (ration is a kind of portion**).
'with' means one lot of letters go next to another.
'ecstasy' becomes 'e' (informal term for the drug).
'in it' means one lot of letters goes inside another.
'adult'+'ration'='adultration'
'adultration' placed around 'e' is 'ADULTERATION'.
'of' is the link.
(Other definitions for adulteration that I've seen before include "Corruption" , "pollution" , "Debasement" .)