In To Kill a Mockingbird, how is Jem "done in by the oldest lawyers trick on record"?
Short answer: By getting Jem to admit to certain actions regarding Boo, Atticus backs Jem into the proverbial corner regarding his other intentions.
What occurs in Chapter 5 of To Kill a Mockingbird is foreshadowed at the end of Chapter 4 as Jem equivocates in response to Dill's asking Jem if they could continue their play at acting out Boo Radley's life after Atticus catches Jem with the scissors,
"I don't know. Atticus didn't say we couldn't--"
"Jem," I said, "I think Atticus knows it anyway."
"No he doesn't . If he did he'd say he did."
I was not so sure, but Jem told me I was being a girl.
Then, in the next chapter Jem still maintains that Atticus has not specifically forbidden them to dramatize Boo's life:
...that Atticus hadn't said we couldn't, therefore we could; and if Atticus ever said we couldn't, Jem had thought of a way around it: he would simply change the names of the characters and then we couldn't be accused of playing anything.
However, Atticus the lawyer beats Jem's artifice. For, when he returns home in order to retrieve a file he has left behind (this file may have been intentionally left at home), He catches the children in the act of trying to leave a note for Boo Radley as Dill finds himself ringing his bell meant to summon Boo directly in the face of Atticus, who tells him to stop his ringing and scold Jem about "tormenting" Boo Radley. Further, he orders Jem to not play any game or ridicule anybody in their neighborhood. Jem protests,
"We weren't makin' fun of him, we weren't laughin' at him...we were just--"
But Atticus finishes Jem's sentence, "putting his life's history on display for the edification of the neighborhood." But, in his denying all else, Jem has so much as admitted to what he has done. By tricking Jem with his scolding, Atticus has caused Jem to concede the only action left, that he was exposing Boo. This is the "oldest lawyer's trick on record."