Certain to keep leaders of every country safe (6)
I believe the answer is:
secure
'safe' is the definition.
('secure' can be a synonym of 'safe')
'certain to keep leaders of every country' is the wordplay.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'to keep' indicates putting letters inside.
'leaders of' suggests taking the first letters.
The initial letters of 'every country' is 'ec'.
'sure' enclosing 'ec' is 'SECURE'.
(Other definitions for secure that I've seen before include "Safe against attack or well fastened" , "land" , "Seize; make fast" , "Safe, protected" , "'Safe, confident (6)'" .)