Certain to keep city safe (6)
I believe the answer is:
secure
'safe' is the definition.
('secure' can be a synonym of 'safe')
'certain to keep city' is the wordplay.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'to keep' is an insertion indicator.
'city' becomes 'ec' (the City of London postcode).
'sure' going around 'ec' is 'SECURE'.
(Other definitions for secure that I've seen before include "Make safe or fasten" , "Safe; fixed" , "Fix; safe" , "Obtain - safe" , "Seize; make fast" .)