Plain juice? Plan rejected (6)
I believe the answer is:
pampas
'plain' is the definition.
(I've seen this before)
'juice? plan rejected' is the wordplay.
'juice?' becomes 'sap' (juice from a plant).
'plan' becomes 'map' (plan can mean a map of something).
'rejected' shows that the letters should be reversed in order.
'sap'+'map'='sapmap'
'sapmap' backwards is 'PAMPAS'.
(Other definitions for pampas that I've seen before include "South American grassland" , "Grassy plains of northern Argentina" , "Argentina grassy plain" , "part of South America" , "Treeless plain(s)" .)