Never corrupt a neophyte (9)
I believe the answer is:
novitiate
'a neophyte' is the definition.
(thesaurus)
'never corrupt' is the wordplay.
'never' becomes 'no' (I've seen this in other clues).
'corrupt' becomes 'vitiate' (synonyms).
'no'+'vitiate'='NOVITIATE'
(Other definitions for novitiate that I've seen before include "New church member" , "newcomer in convent" , "Period in religious life before final vows are taken" , "New convert" .)