The cartoon is set in a costume rental (you could also say costume hire) shop. The owner is speaking on the phone and says, "... but I do have something quite similar." We can see two costumes hanging up - one of Donald Trump (which is marked 'Sold Out') and the other of a scary clown (which is 'In Stock', i.e. available).
We are meant to assume that the person on the other end of the phone is a customer asking if he can rent a Donald Trump costume for Halloween. The joke is that the owner says that the scary clown is quite similar to Trump.
How many Halloween-related things can you spot in the cartoon? (bat, witch's hat, spider, carved pumpkin, skull, black cat).