Plain sweetheart's rejected exercise (6)
I believe the answer is:
steppe
'plain' is the definition.
(I know that steppe is a type of plain)
'sweetheart's rejected exercise' is the wordplay.
'sweetheart's' becomes 'pet's' (both are terms of endearment).
'rejected' is a reversal indicator.
'exercise' becomes 'pe' (Physical Education).
'pets' back-to-front is 'step'.
'step'+'pe'='STEPPE'
(Other definitions for steppe that I've seen before include "Dry grassy plain of eastern Europe and Asia" , "Dry plain of Eurasia" , "Edxtensive plain" , "Large treeless area of Siberia" , "Dry area of grassland in southeast Europe and Siberia" .)