Forbid check on leader of Opposition (4)
I believe the answer is:
veto
'forbid' is the definition.
(I know that forbid can be written as veto)
'check on leader of opposition' is the wordplay.
'check' becomes 'vet' (to vet someone is to check their background).
'on' says to put letters next to each other.
'leader of' says to take the initial letters.
The first letter of 'opposition' is 'o'.
'vet'+'o'='VETO'
(Other definitions for veto that I've seen before include "Overrule, forbid" , "Refuse to endorse" , "A confused vote, altogether negative" , "Power to reject legislation" , "Refuse permission" .)