Formally reject having check on leader of organisation (4)
I believe the answer is:
veto
'formally reject' is the definition.
(to veto is to reject or disallow)
'check on leader of organisation' is the wordplay.
'check' becomes 'vet' (I've seen this before).
'on' means one lot of letters go next to another.
'leader of' says to take the initial letters.
The initial letter of 'organisation' is 'o'.
'vet'+'o'='VETO'
'having' acts as a link.
(Other definitions for veto that I've seen before include "Right to prevent proposed action" , "Authoritative prohibition" , "Don't allow" , "Right to reject a decision" , "Cast over-riding vote" .)