Gym's padded surface (3)
I believe the answer is:
mat
'gym's padded surface' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mat that I've seen before include "Small carpet" , "Thick flat pad on the floor" , "Small protective floor-covering" , "To wipe your feet on" , "Foot-wiper on which welcome may be kept" .)