Actors beginning to exhibit class (5)
I believe the answer is:
caste
'class' is the definition.
(type of social class)
'actors beginning to exhibit' is the wordplay.
'actors' becomes 'cast' (group of actors).
'beginning to' suggests taking the first letters.
The first letter of 'exhibit' is 'e'.
'cast'+'e'='CASTE'
(Other definitions for caste that I've seen before include "Social system" , "Social class or rank" , "Religious or social class" , "Hindu determinant of status" , "Persons with class" .)