Class for actors with energy (5)
I believe the answer is:
caste
'class' is the definition.
(type of social class)
'actors with energy' is the wordplay.
'actors' becomes 'cast' (group of actors).
'with' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
'cast'+'e'='CASTE'
'for' acts as a link.
(Other definitions for caste that I've seen before include "Social station" , "Hereditary social class among Hindus" , "Hindu social rank" , "Hereditary class (in India)" , "Social position" .)