Well, maybe I've typed something into the Custom Preview field just to...you know, see how it looks in a particular font.

But maybe it's some text that others who happen to be using my computer probably shouldn't stumble across.

How do I clear the history of that field?

Maybe you could click on the little link that says Reinitialize, and then maybe the Preview text would be reset. But just maybe, who knows

Just clean "Custom preview" box (and then click "Submit" or push Enter button, of course )

Thanks. Someone told me to clear the cookie for that site. It worked. Thank God!

Yes, of course But you will lose other preferences But yeah, if you want to clean everything, cleaning coockies is the best way.

how can i delete search history from dafont

Yeah, but this is not dafont related. This is browser related.
Just google this "delete autofill entries chrome"