Grassy plains of South America (6)
I believe the answer is:
'grassy plains of south america' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pampas that I've seen before include "Argentinian plains" , "South American prairie" , "Grassy plains of South America" , "Prairies (with grass)" , "Great plains of S America (6)" .)