Tutor disturbed by start of rebellious youth’s initial disloyalty (9)
I believe the answer is:
treachery
'disloyalty' is the definition.
(treachery is a kind of disloyalty)
'tutor disturbed by start of rebellious youth's initial' is the wordplay.
'tutor' becomes 'teacher' (I've seen this before).
'disturbed by' indicates putting letters inside.
'start of' indicates taking the first letters.
'initial' says to take the initial letters.
The first letter of 'rebellious' is 'r'.
The first letter of 'youths' is 'y'.
'teacher' placed around 'r' is 'treacher'.
'treacher'+'y'='TREACHERY'
(Other definitions for treachery that I've seen before include "Treason" , "Perfidy" , "Deceptiveness" , "disloyalty" , "lack of loyalty" .)