Energy-filled, unrestrained exercise (5)
I believe the answer is:
wield
'exercise' is the definition.
(thesaurus)
'energy-filled unrestrained' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'filled' means one lot of letters goes inside another.
'unrestrained' becomes 'wild' ('wild' can be a synonym of 'unrestrained').
'e' placed within 'wild' is 'WIELD'.
(Other definitions for wield that I've seen before include "Use (weapon, authority)" , "exercise" , "Brandish a weapon or exert power" , "Handle - manage" , "Hold and use (a weapon or tool)" .)