Bursts of energy (6)
I believe the answer is:
spasms
'bursts of energy' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for spasms that I've seen before include "Involuntary convulsive movements" , "Convulsions or cramps" , "Cramps or contractions" , "Tics" , "sudden bursts of activity" .)