Upset head of school with hair braid (6)
I believe the answer is:
stress
'upset' is the definition.
(thesaurus)
'head of school with hair braid' is the wordplay.
'head of' indicates taking the first letters.
'with' means one lot of letters go next to another.
'hair braid' becomes 'tress' (I've seen this before).
The initial letter of 'school' is 's'.
's'+'tress'='STRESS'
(Other definitions for stress that I've seen before include "Particular emphasis" , "nervous tension" , "suffering" , "Physical or mental over-exertion" , "Draw attention to" .)