Group about to change seat (5)
I believe the answer is:
caste
'group' is the definition.
(an ethnic or social group)
'about to change seat' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'to change' indicates an anagram.
'seat' anagrammed gives 'aste'.
'c'+'aste'='CASTE'
(Other definitions for caste that I've seen before include "Social station" , "Social division in India" , "Social position" , "Hindu social rank" , "Social status" .)