Group gives actors energy (5)
I believe the answer is:
caste
'group' is the definition.
(an ethnic or social group)
'actors energy' is the wordplay.
'actors' becomes 'cast' (group of actors).
'energy' becomes 'e' (physics symbol as in E=mc2).
'cast'+'e'='CASTE'
'gives' acts as a link.
(Other definitions for caste that I've seen before include "Hindu social rank" , "Level in society" , "Brahmans for instance" , "Social order" , "Hereditary Hindu social division" .)