Watering down of grown-up allowance, including wine ultimately (12)
I believe the answer is:
adulteration
'watering down' is the definition.
(to water something down is to adulterate it)
'grown-up allowance including wine ultimately' is the wordplay.
'grown-up' becomes 'adult' (informal term for an adult).
'allowance' becomes 'ration' (I've seen this before).
'including' is an insertion indicator.
'ultimately' indicates one should take the final letters.
The last letter of 'wine' is 'e'.
'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" .)