University to reject computer operating system (4)
I believe the answer is:
unix
'computer operating system' is the definition.
(I've seen this before)
'university to reject' is the wordplay.
'university' becomes 'u' (abbreviation for university).
'to reject' becomes 'nix' (I've seen this in another clue).
'u'+'nix'='UNIX'
(Other definitions for unix that I've seen before include "See this operating system" , "software" , "Computer open operating system" .)