Stable, balanced (6)
I believe the answer is:
steady
I believe this is a double definition.
'stable' is the first definition.
(similar in meaning)
'balanced' is the second definition.
(I've seen this before)
(Other definitions for steady that I've seen before include "Firmly supported or balanced" , "Not shaking or moving" , "Dependable; unmoving" , "Secure" , "Firm - sweetheart" .)