For his 1973, post-retirement comback album, Ol’ Blue Eyes Is Back, he recorded a Stephen Sondheim tune from a popular musical of the day, A Little Night Music.
"Send in the Clowns" refers to a phrase used in a circus when an unforeseen disaster has occurred, with the clowns being sent in to distract the audience from the problem. The song served as the centerpiece of Ol' Blue Eyes Is Back, and was sort of like that album's "My Way," with autobiographical lyrics such as "Isn't it rich? Isn't it queer, losing my timing this late in my career?". I've wondered if Sinatra didn't select "Send in the Clowns" because he felt a connection to it due to the "sad clown" image used on the Grammy Award winning cover to his 1958 album Frank Sinatra Sings for Only the Lonely.
Here is his performance of it from the 1973 television special, Magnavox Presents Frank Sinatra: