Six agents circling hill station (8)
I believe the answer is:
victoria
'station' is the definition.
(Victoria is a major railway station in London)
'six agents circling hill' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'agents' becomes 'CIA' (Central Intelligence Agency).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'hill' becomes 'tor' (tor is a kind of hill).
'cia' going around 'tor' is 'ctoria'.
'vi'+'ctoria'='VICTORIA'
(Other definitions for victoria that I've seen before include "State < > carriage" , "Old carriage" , "Queen, Australian state and plum" , "She falls in Africa" , "Station - plum - sponge - queen" .)