Six spies guarding hill station (8)
I believe the answer is:
victoria
'station' is the definition.
(Victoria is a major railway station in London)
'six spies guarding hill' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'spies' becomes 'CIA' (Central Intelligence Agency).
'guarding' is an insertion indicator (inserted letters are protected or guarded).
'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 "Monarch" , "Queen - lake -sponge" , "Largest lake in Africa" , "She falls in Africa" , "Queenly" .)