to do something by the rules; to play something in a fair and just manner. John won't play with Bill anymore because Bill doesn't play fair. You moved the golf ball with your foot! That's not playing fair!
Behave honestly and honorably, obey the rules, as in Not every supplier we deal with plays fair, or We can't just leave them to find their own way back-that's not playing fair. Although this idiom conjures up playing by the rules in some game or sport, it actually has been used in this figurative way since the mid-1400s. Also see play the game.