The answer to this riddle/ joke is: It goes back four seconds.
A detailed explanation of this joke (from audioboom.com/Whatsofunny) reads:
The joke tells us that to satisfy its appetite, a clock can go back four seconds. This is where the pun is. Note that “four” is spelled f-o-u-r, as in the number 4. In this case it refers to the seconds of time. To go back in times means to return to the past, or to turn back the hands of time which normally only go in one direction. BUT if the clock goes back for seconds and the word is spelled f-o-r. It completely changes the meaning, to the clock returning to a table or buffet filled with food, in order to get a “second” helping of food. To go back for seconds is a short way of saying this. I don’t know about you but I’m getting hungry just writing this.
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