The default Footnote text style in the blank Word processing template is single spaced 10 pt. Helvetica. If that isn't the default in the template you are working from, you could import that style into your document and apply it to the footnotes.
I am guessing, though, since that's the only default footnote spacing that I've encountered, that you might be talking about the spacing between notes, rather than the line spacing. You can set that to 0 pts. in the Document pane of the inspector.
If Jeff's suggestion solved your footnote challenge, then the following may only be informational.
If you were attempting to format a Chicago (Turabian) Style, single-spaced footnote, then this link provides a free Chicago Style Word (.doc) template download that already has the correct footnote font and spacing set. If you are not writing a thesis or research paper that requires a Chicago Style formatting, then this may not be for you.
You would import this into Pages as a Word template, then save it to the following location as a true Pages template:
your login directory/Library/Application Support/iWork/Pages/Templates/My Templates
instead of My Templates, you could create a new Chicago folder in Templates, then save the Pages Chicago template there.
Now, when starting Pages, the Chicago Style template will appear in the Template Chooser when you need it.