Solid, stable (4)
I believe the answer is:
firm
This clue is a double definition.
'solid' is the first definition.
(I know that solid can be written as firm)
'stable' is the second definition.
(similar in meaning)
(Other definitions for firm that I've seen before include "Company - solid" , "'Solid, unyielding (4)'" , "unlikely to collapse" , "Resolved and steady" , "Steadfast" .)