Sabotaged agenda about Oxford course — not an essential item (9)
I believe the answer is:
appendage
'not an essential item' is the definition.
(a less important feature added on to something)
'sabotaged agenda about oxford course' is the wordplay.
'sabotaged' indicates an anagram.
'about' indicates putting letters inside.
'oxford course' becomes 'PPE' (Philosophy, Politics and Economics degree).
'agenda' is an anagram of 'andage'.
'andage' going around 'ppe' is 'APPENDAGE'.
(Other definitions for appendage that I've seen before include "It hangs on" , "E.g. insect's leg, feeler" , "An outgrowth" , "Thing attached to another" , "Attached item" .)