Ancient school denied guards (5)
I believe the answer is:
olden
'ancient' is the definition.
(as in the olden days)
'school denied guards' is the wordplay.
'guards' says the answer is hidden in the clue.
'OLDEN' is hidden in the letters of 'school denied'.
(Other definitions for olden that I've seen before include "Led on (anag)" , "Out of date" , "of ancient history?" , "Bygone" , "(Of days) in the long distant past!" .)