Energetic person in sack dance (8)
I believe the answer is:
fireball
'energetic person' is the definition.
('fireball' can be a synonym of 'energetic person')
'sack dance' is the wordplay.
'sack' becomes 'fire' ('fire' can be a synonym of 'sack').
'dance' becomes 'ball' (ball is a kind of dance).
'fire'+'ball'='FIREBALL'
'in' is the link.
(Other definitions for fireball that I've seen before include "Energetic type" , "Core of atomic explosion" , "Incendiary projectile" , "Sphere of flame" , "Large bright meteor" .)