Be cooperative (8)
'be cooperative' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for play ball that I've seen before include "Be cooperative" , "perform service in court" , "Co-operate" , "Agree to comply with someone else's rules" , "Most boys love to" .)
(Another definition for playball that I've seen is " Work willingly with others".)