Class act's back in action (5)
I believe the answer is:
caste
'class' is the definition.
(caste is a kind of class)
'act's back in action' is the wordplay.
'back' says to take the final letters.
'in' indicates putting letters inside.
'action' becomes 'case' ('case' can be a synonym of 'action'**).
The final letter of 'act' is 't'.
't' inserted into 'case' is 'CASTE'.
(Other definitions for caste that I've seen before include "Division of Hindu society" , "Hereditary social class in Hindu society" , "Social division in India" , "Social group in India" , "Social class in Hinduism" .)