Attempt to restrain tutor, heading off betrayal (9)
I believe the answer is:
treachery
'betrayal' is the definition.
(I know that betrayal can be written as treachery)
'attempt to restrain tutor heading off' is the wordplay.
'attempt to' becomes 'try' (I've seen this in other clues).
'restrain' means one lot of letters goes inside another.
'tutor' becomes 'teacher' (tutor is a type of teacher).
'heading off' means to remove the first letter.
'teacher' with its initial letter removed is 'eacher'.
'try' enclosing 'eacher' is 'TREACHERY'.
(Other definitions for treachery that I've seen before include "Treason" , "Two-timing" , "Double-dealing" , "Deceptiveness" , "disloyalty" .)