Public school’s disadvantage (8)
I believe the answer is:
downside
This clue is a double definition.
'public school's' is the first definition.
(I've seen this before)
'disadvantage' is the second definition.
(I've seen this before)
(Other definitions for downside that I've seen before include "RC school; bad aspect" , "Drawback" , "public school" , "Somerset RC abbey/college" , "disadvantage" .)