Prefix for active (5)
I believe the answer is:
hyper
'prefix for active' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hyper that I've seen before include "Unable to sit still" , "Prefix meaning excessively, perhaps active" , "Unusually energetic" , "Overexcited" , "Abnormally active" .)