Country house retreat, colonial administrator's first (8)
I believe the answer is:
cliveden
'country house' is the definition.
(I've seen this before)
'retreat colonial administrator's first' is the wordplay.
'retreat' becomes 'den' (both can mean a lair or hideout).
'colonial administrator' becomes 'Clive' (Robert Clive of the East India Company, 1725-1774).
'first' is a charade indicator (letters next to each other).
'den' put after 'clive' is 'CLIVEDEN'.
(Other definitions for cliveden that I've seen before include "mansion used for set" , "Country house" .)