Eternal city's secret operation (7)
I believe the answer is:
lasting
'eternal' is the definition.
(synonyms)
'city's secret operation' is the wordplay.
'city' becomes 'LA' (Los Angeles).
'secret operation' becomes 'sting' (as in a police sting).
'la'+'sting'='LASTING'
(Other definitions for lasting that I've seen before include "Permanent" , "Enduring" , "Evergreen" , "long-term" , "there for long while" .)