Outside ancient city, reclaim ground that's unstable (9)
I believe the answer is:
mercurial
'unstable' is the definition.
(thesaurus)
'outside ancient city reclaim ground' is the wordplay.
'outside' indicates putting letters inside.
'ancient city' becomes 'ur' (ancient Sumerian city state).
'ground' is an anagram indicator ('grind' the letters into a new form).
'reclaim' with letters rearranged gives 'mercial'.
'ur' put inside 'mercial' is 'MERCURIAL'.
'that's' is the link.
(Other definitions for mercurial that I've seen before include "Prone to sudden mood swings" , "Changeable in mood" , "Changing unpredictably" , "Sprightly" , "Temperamental" .)