Stable; fixed (6)
I believe the answer is:
steady
I believe this clue is a double definition.
'stable' is the first definition.
(similar in meaning)
'fixed' is the second definition.
(I've seen this before)
(Other definitions for steady that I've seen before include "Stable, not shaky" , "Firm or constant" , "Not wavering" , "Stable or regular" , "Firmly supported or balanced" .)