Steady but, being old, about to start badly (6)
I believe the answer is:
stable
'steady' is the definition.
('stable' can be similar in meaning to 'steady')
'being old about to start badly' is the wordplay.
'being old' becomes 'stale' ('stale' can be a synonym of 'old'**).
'about' means one lot of letters goes inside another.
'to start' says to take the initial letters.
The initial letter of 'badly' is 'b'.
'stale' going around 'b' is 'STABLE'.
'but' acts as a link.
(Other definitions for stable that I've seen before include "Horse barn" , "Steady in the outhouse" , "Steady in the horse-house" , "Firmly-fixed; building" , "Animal accommodation is steady" .)