How can I quickly clear cookies for a specific site using Safari for Mac?

There you go!

Clearing a Specific Cookie on Safari: Safari makes you do a little more mousework to get to the listing of cookies in the system, and you have to specify which you want to remove. To get started, go to the preferences menu, and open the “Security” Options. Partway down the page is a button labeled “Show Cookies.” Click this.

Once you have clicked the “Show Cookies” button, you will be shown a listing of all of the cookies your browser has stored, along with a handy search box. Type the current domain name into this search to see a listing of cookies set by the current site. From here, you can select and delete the cookies.

As @user6297 says in his comment, the above answer no longer works as of El Capitan. You now need to go to Preferences > Privacy > Details as shown in the image. Note that the Details button might take a moment to appear.

Then enter the domain in the Search box, select the item you want to remove, and click Remove.