About to support popular South American (4)
I believe the answer is:
inca
'south american' is the definition.
(the Inca were a South American civilization)
'about to support popular' is the wordplay.
'about' becomes 'ca' (abbreviation for 'circa').
'to support' says to put letters next to each other (in a down clue, some letters hold others up).
'popular' becomes 'in' ('in' can mean trendy).
'ca' put after 'in' is 'INCA'.
(Other definitions for inca that I've seen before include "Atahualpa was" , "Old Peruvian descended from Cain" , "Old Peruvian people" , "Member of ancient Peruvian tribe" , "who lived in S America" .)