To co-operate, take part in a game before social function (4,4)
I believe the answer is:
play ball
'to co-operate' is the definition.
(I've seen this before)
'take part in a game before social function' is the wordplay.
'take part in a game' becomes 'play' (I've seen this in another clue).
'before' says to put letters next to each other.
'social function' becomes 'ball' (I've seen this before).
'play'+'ball'='PLAY BALL'
(Other definitions for play ball that I've seen before include "Do what is needed" , "go along" , "meet a delivery?" , "Co-operate" , "Be cooperative" .)