Steady but, being old, about to start badly (6)
I believe the answer is:
stable
'steady' is the definition.
(similar in meaning)
'being old about to start badly' is the wordplay.
'being old' becomes 'stale' ('stale' can be a synonym of 'old').
'about' indicates putting letters inside.
'to start' says to take the initial letters.
The first 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" .)