Unusually energetic (5)
I believe the answer is:
hyper
'unusually energetic' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hyper that I've seen before include "Excessively, active, say" , "Really excited" , "Unable to sit still" , "Prefix meaning 'Excessive'" , "Overexcited" .)