Fidget terribly, being talented (6)
I believe the answer is:
gifted
'being talented' is the definition.
('gifted' can be a synonym of 'talented')
'fidget terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'fidget' with letters rearranged gives 'GIFTED'.
(Other definitions for gifted that I've seen before include "Possessed of natural talent" , "Exceptionally clever" , "exceptional talent" , "Talented, having great natural ability" , "Brilliant" .)