Film relating to school (4)
I believe the answer is:
eton
'to school' is the definition.
(famous UK public school)
'film relating' is the wordplay.
'film' becomes 'et' (E.T. the Extra-Terrestrial).
'relating' becomes 'on' (I've seen this in other clues).
'et'+'on'='ETON'
(Other definitions for eton that I've seen before include "Leading English public school" , "School near Slough" , "College, collar or crop" , "Noted public school" , "Public school founded by Henry VI" .)