Certain to keep city safe (6)
I believe the answer is:
secure
'safe' is the definition.
(synonyms)
'certain to keep city' is the wordplay.
'certain' becomes 'sure' (synonyms).
'to keep' is an insertion indicator.
'city' becomes 'EC' (postcode for the City of London).
'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" .)