Disallow cipher, following check (4)
I believe the answer is:
veto
'disallow' is the definition.
('veto' can be a synonym of 'disallow')
'cipher following check' is the wordplay.
'cipher' becomes 'o' (old-fashioned term for zero).
'following' is a charade indicator (letters next to each other).
'check' becomes 'vet' (I've seen this before).
'o' put after 'vet' is 'VETO'.
(Other definitions for veto that I've seen before include "Don't allow" , "Overrule, forbid" , "Embargo" , "Authoritative prohibition" , "Red light" .)