Variable stayed constant (6)
I believe the answer is:
steady
'constant' is the definition.
(synonyms)
'variable stayed' is the wordplay.
'variable' indicates an anagram (a varied form of the letters).
'stayed' anagrammed gives 'STEADY'.
(Other definitions for steady that I've seen before include "not easily shaken" , "Stable and reliable" , "Not wobbly" , "Poised" , "Not liable to fluctuate" .)