Head of eighteenth century school (4)
I believe the answer is:
eton
'school' is the definition.
(famous UK public school)
'head of eighteenth century' is the wordplay.
'head of' suggests taking the first letters.
'century' becomes 'ton' (both can mean one hundred of something).
The initial letter of 'eighteenth' is 'e'.
'e'+'ton'='ETON'
(Other definitions for eton that I've seen before include "Town opposite Windsor on the Thames" , "Public school near Windsor" , "Leading English public school" , "British public school of note" , "School near Slough" .)