House firm on double time - a case of granite! (7)
I believe the answer is:
cottage
'house' is the definition.
(cottage is a kind of house)
'firm on double time a case of granite' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'on' is a charade indicator (letters next to each other).
'double' indicates the doubling of some letters.
'time' becomes 't'.
'case of' says to hollow out the word (remove centre letters) (outsides of).
'granite' with its centre removed is 'ge'.
't' duplicated is 'tt'.
'co'+'tt'+'a'+'ge'='COTTAGE'
(Other definitions for cottage that I've seen before include "Rural house" , "Small house, in the country say" , "Small home" , "simple accommodation" , "bungalow" .)