This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / Tinker, Tailor, Soldier, Spy
Smiley's promise to Ricky Tarr that he will do his best to get her back, when the viewer knows this is hopeless, and Smiley strongly suspects as well.
Jim Prideaux's story arc. Shot down in Hungary, brutally tortured in Russia, then sent to work as a schoolmaster in England, where he meets a nice Morality Pet student, only to push him away. Finally he learns for certain that his best friend/maybe lover Bill Haydon is the mole, and kills him.
In the mini-series, Smiley's interview with Connie Sachs.
"Poor loves. Trained to Empire, trained to rule the waves. Englishmen could be proud then. They could, George. All gone, taken away. Bye-bye world. If it's bad, George, don't come back. Promise? I want to remember you, just as you were. My lovely, lovely boys..."
Due to the possibility he may be monitored by the Circus, Guillam is forced to kick out his gay live-in partner without so much as an explanation.
In the film, after Haydon is caught and Smiley interrogates him, he denies that he's Karla's "lapdog." Smiley raises his voice for the only time in the entire film and screams "WELL, WHAT ARE YOU THEN?" A stunning bit of performance from Gary Oldman letting out all the emotion he'd been holding in the past two hours.