You are working against the clock when you are trying to finish your work within a limited amount of time. Example: "We worked against the clock all day to get that report done by five."

You often compare the time on the clock against how much work you have left to do when you are working against the clock. Example: "We have to finish this report by 8 o'clock tonight, so we're really working against the clock."

The passing of time ("the clock") works against you when you are working against the clock. Example: "Mom, will you have time to help me with my homework today?" Reply: "Sorry, I won't. I'll be working against the clock to finish my presentation."