Okay, the clunk is coming from your drivetrain. This is because as you slow down to stop with the engine at a raised idle, it is pulling against the tranny, and when the tranny shifts back into first it causes the clunk. If you shift the tranny into first and get the car up to about 20 mph, you will likely get the same clunk by lightly getting on and off the throttle.

The slack is either in your driveshaft or your differential. Do you have or have you had any vibrations under load at any particular speed? This usually indicates bad u-joints (along with clunks). Any whining sounds from the diff (gears) or grinding while cornering (side gear wear)?

Anyways, the proper way to find out is to get the car up on jackstands and with the engine off and the tranny in park, find out where the slack is by moving one rear wheel while someone else holds the other one still and looking carefully at the u-joints. If they are good, note about how many degrees of movement (slack) you have on the wheel you're turning.

Do you feel the clunk through the shifter more than the steering wheel or less? Like Jim says, there could still be other problems, but I'd check the driveshaft and diff for now.