Head of school to finish under peak salary (7)
I believe the answer is:
stipend
'salary' is the definition.
(thesaurus)
'head of school to finish under peak' is the wordplay.
'head of' suggests taking the first letters.
'to finish' becomes 'end' ('end' can be a synonym of 'finish'**).
'under' says to put letters next to each other.
'peak' becomes 'tip' ('tip' can be a synonym of 'peak').
The first letter of 'school' is 's'.
'end' put after 'tip' is 'tipend'.
's'+'tipend'='STIPEND'
(Other definitions for stipend that I've seen before include "Clergyman's salary" , "Salary, allowance" , "Parson's pay" , "Wage" , "Fixed salary" .)