Thereupon, chose to incorporate simple garment (6)
I believe the answer is:
poncho
'simple garment' is the definition.
(I've seen this before)
'thereupon chose to incorporate' is the wordplay.
'to incorporate' says the answer is hidden in the clue (I've seen 'incorporates' mean this).
'PONCHO' is hidden within 'thereupon chose'.
(Other definitions for poncho that I've seen before include "South American cape" , "Kind of cloak like blanket with slit for head" , "Overgarment with hole for head" , "South American cloak with slit for the head" , "Blanket-style cloak" .)