Ban cheat in writing (9)
I believe the answer is:
proscribe
'ban' is the definition.
(thesaurus)
'cheat in writing' is the wordplay.
'cheat' becomes 'crib' (cribbing is a kind of cheating).
'in' means one lot of letters goes inside another.
'writing' becomes 'prose' (I've seen this in other clues).
'crib' put inside 'prose' is 'PROSCRIBE'.
(Other definitions for proscribe that I've seen before include "Ban" , "Prohibit" , "Forbid; condemn" , "Denounce" , "Exile" .)