Elite British boys' school (4)
I believe the answer is:
eton
'elite british boys' school' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eton that I've seen before include "Collar - crop - jacket - school" , "Public school (sounds like 20)" , "Independent school" , "Berkshire town" , "Posh English school of note" .)