Equilibrium unaltered supporting edges of seat (6)
I believe the answer is:
stasis
'equilibrium' is the definition.
(I've seen this before)
'unaltered supporting edges of seat' is the wordplay.
'unaltered' becomes 'asis' (ie something left 'as is').
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'edges of' suggests removing the centre.
'seat' with its middle taken out is 'st'.
'asis' put after 'st' is 'STASIS'.
(Other definitions for stasis that I've seen before include "Period of equilibrium" , "Stoppage of blood circulation" , "lack of action" , "Standstill" , "State of immobility or inactivity" .)