Vitality plus unusual energy (5)
I believe the answer is:
pulse
'vitality' is the definition.
(I've seen this before)
'plus unusual energy' is the wordplay.
'unusual' indicates anagramming the letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'plus' with letters rearranged gives 'puls'.
'puls'+'e'='PULSE'
(Other definitions for pulse that I've seen before include "Rhythmical throb of arteries" , "Vibration" , "runner, possibly" , "Beat or throb of heart" , "Beat - lentil" .)