If you would like to specify the use of a different font in xclock and can't get the font to display properly, try specifying the -norender parameter. The following is taken from the manpage.
font (class Font)
Specifies the font to be used for the digital clock. Note that
variable width fonts currently will not always display correctly. This font is only used when Xrender is not in effect.
Also try specifying the new font with -face rather than -font.