Plain foot movement needed with gym (6)
I believe the answer is:
steppe
'plain' is the definition.
(I know that steppe is a type of plain)
'foot movement needed with gym' is the wordplay.
'foot movement' becomes 'step' (you take steps with your feet).
'needed with' is a charade indicator (letters next to each other).
'gym' becomes 'pe' (short for Physical Education).
'step'+'pe'='STEPPE'
(Other definitions for steppe that I've seen before include "Large area of flat grassland in Europe or Siberia" , "Large treeless area of Siberia" , "Dry area of grassland in southeast Europe and Siberia" , "crosses Siberia" , "plain territory" .)