Country house's grand spread (6)
I believe the answer is:
grange
'country house's' is the definition.
(I've seen this before)
'grand spread' is the wordplay.
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'spread' becomes 'range' (I've seen this in other clues).
'g'+'range'='GRANGE'
(Other definitions for grange that I've seen before include "Ganger in country house with farm buildings" , "Country house with outbuildings" , "Country-house" , "Country house and surrounding buildings" , "Farmhouse" .)