Corrupt grown-up English count (10)
I believe the answer is:
adulterate
'corrupt' is the definition.
(thesaurus)
'grown-up english count' is the wordplay.
'grown up' becomes 'adult' (I've seen this before).
'english' becomes 'e' (abbreviation).
'count' becomes 'rate' (I've seen this in other clues).
'adult'+'e'+'rate'='ADULTERATE'
(Other definitions for adulterate that I've seen before include "Debase by mixing with something inferior" , "Water down" , "Spoil" , "Doctor" , "Spike" .)