The speaker imagines eating a clock for breakfast, and the poem treats this absurd act with a calm, matter-of-fact tone. Everything gets swallowed: the noise of ticking ("the din"), the hands you can almost feel, even the electrical cord. The title, "Diet," suggests a daily routine, as if consuming time were just another way of looking after yourself. Calling the meal "a brave cake" is funny, but it also admits that this takes nerve, since the clock is hard, noisy and not meant to be digested.
The last line shifts the mood. After the strange meal, the speaker is "still." The word can mean quiet and motionless, but it can also mean "even now," so the speaker may be both at peace and unchanged. Judging by the imagery, the poem seems to be about wanting relief from the pressure of schedules and passing time. The feeling is playful on the surface, but underneath there is a quiet longing for silence, and the ending leaves that calm hanging in the air.