Criminal seen round ancient city? Make certain (6)
I believe the answer is:
ensure
'make certain' is the definition.
('ensure' can be a synonym of 'make certain')
'criminal seen round ancient city?' is the wordplay.
'criminal' indicates an anagram.
'round' indicates putting letters inside.
'ancient city?' becomes 'ur'.
'seen' is an anagram of 'ense'.
'ense' placed around 'ur' is 'ENSURE'.
(Other definitions for ensure that I've seen before include "Make certain that (something occurs)" , "See run (anag)" , "Make certain for the future" , "Make certain, clinch" , "Make certain, secure" .)