Play around beside mattress (5)
I believe the answer is:
futon
'mattress' is the definition.
(futon is a kind of mattress)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for futon that I've seen before include "Foldable mattress" , "A Japanese folding bed-chair" , "Japanese matress-bed" , "Japanese sofa bed" , "Sleep on it" .)