Disclose when head departs college (4)
I believe the answer is:
eton
'college' is the definition.
(Eton College, English private school)
'disclose when head departs' is the wordplay.
'disclose' becomes 'let on' (as in disclosing or letting on about a secret).
'when head departs' means to remove the first letter.
'leton' with its first letter taken away is 'ETON'.
(Other definitions for eton that I've seen before include "English college of note" , "learn here?" , "Noted public school" , "old school" , "Posh British public school" .)