Unable to settle balance? Not so (8)
I believe the answer is:
restless
'unable to settle' is the definition.
(I've seen this before)
'balance? not so' is the wordplay.
'balance?' becomes 'rest' (I've seen this before**).
'not so' becomes 'less' (I've seen this before).
'rest'+'less'='RESTLESS'
(Other definitions for restless that I've seen before include "Unsettled, fidgety" , "Wakeful" , "Constantly moving about or fidgeting" , "Fidgety - unable to sleep" , "Uneasy, unable to relax" .)