Bishop in old farm building (6)
I believe the answer is:
stable
'farm building' is the definition.
(stable is a kind of farm building)
'bishop in old' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'in' means one lot of letters goes inside another.
'old' becomes 'stale' (similar in meaning).
'b' put into 'stale' is 'STABLE'.
(Other definitions for stable that I've seen before include "Outhouse for horses" , "Jesus was born in one" , "Well balanced" , "firmly tied on?" , "far from bats?" .)