East Side set ultimately not the most stable (9)
I believe the answer is:
steadiest
'the most stable' is the definition.
(I've seen this in another clue)
'east side set ultimately not' is the wordplay.
'set' indicates anagramming the letters (instruction to set the letters).
'ultimately' says to take the final letters.
The last letter of 'not' is 't'.
'east'+'side'='eastside'
'eastside' anagrammed gives 'steadies'.
'steadies'+'t'='STEADIEST'
(Other definitions for steadiest that I've seen before include "Best-balanced -- it's sedate (anag)" , "Most secure" , "Most reliable" .)