A juggler is an entertainer that tosses several objects up into the air and catches them. Juggler sounds like "jugular" which is an adjective meaning of the throat. Jugular is also known to be short for "jugular vein" which is an important vein found in the neck. If the jugular vein is cut, a person can bleed out and die. The joke is saying that in order to kill a circus in one blow, go for the juggler which sounds like jugular. The joke is a play on the word juggler.
To kill a circus in one blow , go for the juggler .