Companion with cake failing to open large country house (7)
I believe the answer is:
chateau
'large country house' is the definition.
'companion with cake failing to open' is the wordplay.
'companion with' becomes 'ch' (Companion of Honour. I am not sure about the 'with' bit.).
'cake' becomes 'gateau' (gateau is a kind of cake).
'failing to open' means to remove the first letter (the opening letter is removed).
'gateau' with its initial letter taken away is 'ateau'.
'ch'+'ateau'='CHATEAU'
(Other definitions for chateau that I've seen before include "French palace" , "Stately home in France" , "French wine producer" , "Vineyard estate around Bordeaux" , "Loire Valley attraction" .)