On balance, not so fidgety (8)
I believe the answer is:
restless
'fidgety' is the definition.
(synonyms)
'on balance not' is the wordplay.
'on' says to put letters next to each other.
'balance' becomes 'rest' (I've seen this in another clue).
'not' becomes 'less' (I've seen this in other clues).
'rest' put next to 'less' is 'RESTLESS'.
'so' acts as a link.
(Other definitions for restless that I've seen before include "Fidgetty" , "Ceaselessly in motion" , "Fidgety, uneasy" , "Fidgety - unable to sleep" , "Unable to relax or be still" .)