Place cheese in 1-quart casserole or bowl.
Microwave 1 to 2 minutes on '6', or until softened.
Drain clam liquid into casserole.
Add horseradish and onion, mix thoroughly.
Microwave 1 to 2 minutes on HIGH, or until hot.
For ovens without solid state heat control, Microwave 2 minutes to 2 minutes, 30 seconds on '5'.