You can probably do that IF you've got enough pins available. The Clock displays I've seen use, typically, 7 pins per digit, and a backplane oscillator common to all segments. There's also a colon between hours and minutes, and between minutes and seconds, which you're at liberty to use or not.

You will probably get a satisfactory display if you drive the backplane with the compliment of the level to which you drive active segments, assuming, of course, that your clock display is formatted in the usual 7-segment format. How good the contrast is will depend somewhat on the specifics of the display. If need be, you can drive the backplane with a spare RS-232 driver, if it's available. That will help with contrast.

I've seen this done in several ways. Assuming you're using a 3-volt part, you can drive the active segments, the value of which you probably will look up in a table, and when you do, you drive the segment high when the backplane is low, and drive the segments low when the backplane is high. This assumes a more-or-less 50% duty cycle on the backplane drive. Another way, of course, can be accomplished by driving the backplane with a full voltage swing, while driving the segments statically through a voltage divider to half the backplane potential difference, i.e. drive active segments to 1.5 volts while driving the backplane and inactive segments with that ~50% duty-cycle oscillator at about 40-80 Hz.

Since this uses so many pins per digit, I prefer to use a CPLD to drive the display, and, when I'm doing that, I simply XOR the positive-logic active segments with the backplane drive oscillator, all from a single supply, between 3 and 5 volts.