To play ball

To agree to work with or help someone in the way they have suggested

e.g. the police officer told the man accused of theft that if he co-operated with the investigation, answered truthfully and pleaded guilty at the earliest opportunity that they would get a much lighter sentence so he decided to play ball.

The (non-sporting) use - to agree to work with or help someone in the way they have suggested - is first recorded, according to the Oxford English Dictionary in a 1903 novel, Back to the Woods, by Hugh McHugh (pen name of George Hobart): “Well, if Bunch should refuse to play ball I could send the check back to Uncle Peter.