As with most things in life, I get the joke…
…only because the Simpsons spoofed it. Those in the Simpsons generation, unite!
It’s a stock phrase from the song “Send in the clowns” from the musical “A Little Night Music”. Judy Collins popularized in the '70s (and I saw her sing it in March in New York!).
EDIT: Didn’t see your edit! It’s like “My Way” where you might have never seen the actual original usage but the song gets integrated into pop culture
If it makes you feel any better, had I been there, I would have looked around the room and said, “Don’t bother, they’re here.”
Isn’t that rich?
Don’t you approve?
I also pictured Krusty’s Comeback Special, and have no knowledge of the original source.
But I still laughed.
The OP made me smile – this made me giggle uncontrollably.
Bummer, dude. FWIW, I would have roared.
For the record, that’s “Entrance of the Gladiators” (YouTube link).
The other day I had my own ancient pop-culture reference go over the heads of my younger co-workers. A PC with a noisy fan was making a racket so I thumped it with my fist, the noise stopped, and gave my best Fonzie “ayyyy”.
crickets
I told them if they were 15 years older that would’ve been funny.
No – after you say “Send them in, you have to say"There ought to be clowns” first.
Then (preferably after they’ve come in) you get to say “Don’t bother. They’re here.”
“Alas, Rosencrantz, the knife is serrated, and rusty, and it pains me so” - Guildenstern*
- = no. not really.
OP here.
Well, thanks everyone. I do feel better reading your responses; I knew you’d come through.
I never accept the ‘generational thing’ argument, though. Had someone made a reference to, say, the WWI song ‘Over There’, I certainly would have understood the remark. And I most definitely was not around in the 19-teens. 
As for this:
Brilliant!
mmm
Ah, of course. (But is it just me that has a hard time associating the bouncy fairground organ tune with gladiators?)
I love you guys. One Halloween, I went to work with what I thought was a great costume. I was dressed in all black with a black hood, carried around a stuffed cat on a noose, and wore a name tag that said, “Hello, my name is Curiosity.” People Did Not Get It! I had to actually say several times: “Hello! My name is Curiosity! I Killed The Cat!”
One more: My sister used to work in a hospital. One day, two patients sharing a room had the surnames Unger and Madison. And they were roommates! She pointed out to a group of her co-workers how amusing this was, then promptly had to explain why it was amusing. Nobody got the reference.
That’s odd…
To the OP: I would have just sat there humming. And they still wouldn’t have gotten it.
I launched a dud just a few minutes ago. My coworker emails me this story:
I said: I guess the bull found him unpalatable. ::Crickets:: Aw, COME ON PEOPLE!
Put me down for that one as well. 1974 musical? What?
She told a couple nurses… what an odd couple…
Hey, you.
Get offa my clown.