Equilibrium unaltered on edges of seat (6)
I believe the answer is:
stasis
'equilibrium' is the definition.
(thesaurus)
'unaltered on edges of seat' is the wordplay.
'unaltered' becomes 'asis' (I've seen this in other clues).
'on' is a charade indicator (letters next to each other).
'edges of' says to hollow out the word (remove centre letters).
'seat' with its middle taken out is 'st'.
'asis' put after 'st' is 'STASIS'.
(Other definitions for stasis that I've seen before include "Secret police force's" , "lack of activity" , "Standstill" , "Total lack of movement" , "State of inactivity where nothing moves" .)