Could be the brake pad(s) binding or the front suspension hitting the turn stops which limit the amount of steering travel. A wee dab of grease on the stops could help if that is the case.

Does this happen only after having applied the brakes and moving from a standing stop?

Does this happen either in a right or left hand turn or either/both?

Does it happen when you are standing still (turn wheel full when standing) or rolling.

If rolling, how fast do you have to go to hear the clunk? And will it repeat?

If rolling, does this happen only when the car is "cold" or will it occur when the car is warmed up and at operating temp?

Have you checked your power steering fluid level?

Answers to these kinds of questions could help narrow the guess...