See court spies from abroad crossing ancient city (5)
I believe the answer is:
curia
'see court' is the definition.
(court of the Holy See)
'spies from abroad crossing ancient city' is the wordplay.
'spies from abroad' becomes 'CIA' (CIA in the US).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'ancient city' becomes 'ur' (ancient Sumerian city state).
'cia' going around 'ur' is 'CURIA'.
(Other definitions for curia that I've seen before include "Central church administration in the Vatican" , "Church organisation" , "Court of the papal sea" , "Governing body of the Roman Catholic church" , "Papal court, Vatican government" .)