Kid's companion put down floor covering in gym (8)
I believe the answer is:
playmate
'kid's companion' is the definition.
(I've seen this in another clue)
'put down floor covering in gym' is the wordplay.
'put down' becomes 'lay' (lay something down).
'floor covering' becomes 'mat' (a mat is a floor covering).
'in' indicates putting letters inside.
'gym' becomes 'pe' (short for Physical Education).
'lay'+'mat'='laymat'
'laymat' put within 'pe' is 'PLAYMATE'.
(Other definitions for playmate that I've seen before include "Child's friend" , "Chum in recreation" , "My palate (anag.)" , "fun chum" , "Companion for fun" .)