One fool undermining university (4)
I believe the answer is:
unit
'one' is the definition.
('unit' can be a synonym of 'one')
'fool undermining university' is the wordplay.
'fool' becomes 'nit' (I've seen this before).
'undermining' says to put letters next to each other.
'university' becomes 'u'.
'nit' put after 'u' is 'UNIT'.
(Other definitions for unit that I've seen before include "Single whole as part of a larger whole" , "Group that's part of a larger organisation" , "Single item, maybe part of a set" , "Subdivision of a whole" , "Kitchen element?" .)