Tutor longing to get through endless time at school (7)
I believe the answer is:
teacher
'tutor' is the definition.
(tutor is a type of teacher)
'longing to get through endless time at school' is the wordplay.
'longing' becomes 'ache' (longing can mean aching or pining for something).
'to get through' is an insertion indicator.
'endless' means to remove the last letter.
'time at school' becomes 'term' (school term).
'term' with its last letter removed is 'ter'.
'ache' put into 'ter' is 'TEACHER'.
(Other definitions for teacher that I've seen before include "'Instructor, educator (7)'" , "Schoolmaster" , "CARRY ON FILM" , "Coach" , "Member of school staff" .)