Class actors have energy (5)
I believe the answer is:
caste
'class' is the definition.
(type of social class)
'actors have energy' is the wordplay.
'actors' becomes 'cast' (group of actors).
'have' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'cast'+'e'='CASTE'
(Other definitions for caste that I've seen before include "Social division in India" , "Hindu determinant of status" , "Hereditary Hindu social division" , "Social division in Hinduism" , "Social class in India" .)