Bishop taken in by old firm (6)
I believe the answer is:
stable
'firm' is the definition.
(similar in meaning)
'bishop taken in by old' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'taken in by' is an insertion indicator.
'old' becomes 'stale' (similar in meaning).
'b' going within 'stale' is 'STABLE'.
(Other definitions for stable that I've seen before include "Steady in the horse house" , "Steadfast, resolute" , "It might accommodate Arab" , "Horse's house" , "'Steady, not given to sudden change (6)'" .)