The cooperative thing to play (4)
I believe the answer is:
ball
'play' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ball that I've seen before include "Social function for dancing" , "Spherical object" , "Throw a dance" , "Spherical plaything" , "Sphere - dance" .)