Social stratum (5)
I believe the answer is:
caste
'social stratum' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for caste that I've seen before include "Division of Hindu society" , "Social station" , "Hindu determinant of status" , "Hereditary social class in Hindu society" , "Large, long-necked, wading bird" .)