Farm building, not fresh, with black interior (6)
I believe the answer is:
stable
'farm building' is the definition.
(I know that stable is a type of farm building)
'not fresh with black interior' is the wordplay.
'not fresh' becomes 'stale' (stale is the opposite of fresh).
'with' is an insertion indicator.
'black' becomes 'sable' (heraldic term for black).
'interior' indicates the central letters.
The central letter of 'sable' is 'b'.
'stale' going around 'b' is 'STABLE'.
(Other definitions for stable that I've seen before include "Steady place for a horse?" , "Equine housing" , "Horse barn" , "firmly tied on?" , "Durable; horse shed" .)