Change horrifies head girl (5)
I believe the answer is:
edith
'girl' is the definition.
(I've seen this before)
'change horrifies head' is the wordplay.
'change' becomes 'edit' (edit can mean to change or alter).
'head' says to take the initial letters.
The first letter of 'horrifies' is 'h'.
'edit'+'h'='EDITH'
(Other definitions for edith that I've seen before include "and 19 Across: 'Little Sparrow'" , "Cavell, Piaf or Wharton" , "- Cavell, nurse executed in 1915" , "Woman's name" , "Eg Cavell" .)