Don't start to repair roof? Silly — not part of house to leave open! (5,4)
I believe the answer is:
front door
'not part of house to leave open' is the definition.
(a front door should not be left unlocked)
'don't start to repair roof? silly' is the wordplay.
'start to' says to take the initial letters.
'silly' indicates anagramming the letters.
The first letter of 'repair' is 'r'.
'dont'+'r'+'roof'='dontrroof'
'dontrroof' anagrammed gives 'FRONT DOOR'.
(Other definitions for front door that I've seen before include "Exit" , "Formal entrance to house" , "main entrance" .)