An argument about the first social order (5)
I believe the answer is:
caste
'social order' is the definition.
(I've seen this in another clue)
'an argument about the' is the wordplay.
'an argument' becomes 'case' (as in making one's case).
'about' is an insertion indicator.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'case' enclosing 't' is 'CASTE'.
'first' is the link.
(Other definitions for caste that I've seen before include "Religious or social class" , "Persons with class" , "Social order" , "Hindu social rank" , "Division of Hindu society" .)