Favourite’s rejected exercise; it’s plain (6)
I believe the answer is:
steppe
'plain' is the definition.
(steppe is a kind of plain)
'favourite's rejected exercise' is the wordplay.
'favourite's' becomes 'pets' ('pet' can be a synonym of 'favourite').
'rejected' says the letters should be written in reverse.
'exercise' becomes 'pe' (Physical Education).
'pets' back-to-front is 'step'.
'step'+'pe'='STEPPE'
'it's' is the link.
(Other definitions for steppe that I've seen before include "Large treeless area of Siberia" , "Large area of flat grassland in Europe or Siberia" , "Edxtensive plain" , "Large area of unforested grassland" , "Dry area of grassland in southeast Europe and Siberia" .)