Island school with university entrance (4)
I believe the answer is:
guam
'island' is the definition.
(I know that Guam is a island)
'school with university entrance' is the wordplay.
'school' becomes 'gam' (I've seen this in other clues).
'with' indicates putting letters inside.
'entrance' says to take the initial letters.
The first letter of 'university' is 'u'.
'gam' placed around 'u' is 'GUAM'.
(Other definitions for guam that I've seen before include "North Pacific island, base for the US navy and air force" , "US island east of the Philippines" , "place in the Marianas" , "NW Pacific island with large US base" , "Island in the West Pacific" .)