I'm leaving high grade university, securing pass (5)
I believe the answer is:
adieu
'i'm leaving' is the definition.
(farewell)
'high grade university securing pass' is the wordplay.
'high grade' becomes 'A' (academic grade).
'university' becomes 'U' (abbreviation for university).
'securing' indicates putting letters inside.
'pass' becomes 'die' (as in passing away).
'a'+'u'='au'
'au' enclosing 'die' is 'ADIEU'.
(Other definitions for adieu that I've seen before include "Farewell, adios" , "Auf wiedersehen" , "Some say it when leaving" , "Goodbye, mon ami!" , "left, making this?" .)