Universal veto for operating system (4)
I believe the answer is:
unix
'for operating system' is the definition.
(Unix is an operating system)
'universal veto' is the wordplay.
'universal' becomes 'u'.
'veto' becomes 'nix' (nix can mean to veto or cancel).
'u'+'nix'='UNIX'
(Other definitions for unix that I've seen before include "software" , "Computer open operating system" , "See this operating system" .)