His best friend John couldn't put up with Pat always revealing his secrets, so decided to play a trick on him.

play 在这里是动词，意思是"进行、实施"某种行为或活动。 a 是不定冠词，表示"一个"。 trick 是名词，意思是"诡计、恶作剧"。 on 是介词，表示动作的对象或目标。 play a trick on 是一个固定短语，意思是"对某人搞恶作剧"或"捉弄某人"。