Not easily, though I think you could script it and bind a key to such a script.
But why do you want to? Normally when you hit Cmd/Ctrl-F, the Find what field is highlighted, so you can just start typing. And then if you hit TAB, then the Change to field is highlighting, you can type there.
What's your application?
Not sure I understand (or that John does). I'm thinking you mean you want to remove some of the previous strings you entered, but leave others for easy selection in the find and change dropdowns. A little clarification would help.
Restarting ID should clear the lists entirely -- not necessarily what you wanted, but if there are some strings you want to preserve you can save them as queries accessible from the dropdown at the top of the dialog.
Thanks, John and Peter, for taking the time to reply.
Why do I wish to clear previous text strings: just obsessive housecleaning, I suppose.
Restarting ID doesn't clear them. And it's not worth the bother to trash InDesign Defaults or InDesign SavedData.
And I know that I can save queries.
Again, thanks. This is a small issue in a hugely capable program.