Debase grown-up with fifth-rate charge? (10)
I believe the answer is:
adulterate
'debase' is the definition.
(synonyms)
'grown-up with fifth-rate charge?' is the wordplay.
'grown-up' becomes 'adult' (informal term for an adult).
'with' says to put letters next to each other.
'fifth-rate' becomes 'E' (fifth worst category).
'charge?' becomes 'rate' (rate is a kind of charge).
'adult'+'e'+'rate'='ADULTERATE'
(Other definitions for adulterate that I've seen before include "Reduce purity" , "Debase by mixing with something inferior" , "Water down" , "Spoil" , "Corrupt" .)