From the Apple Help article:
Do one of the following:
Choose Clip > Show Video Animation (or press Control-V) to see video effects or choose Clip > Show Audio Animation to see audio effects, select the effect, and press Delete.
Select the effect in the Video inspector or Audio inspector, and press Delete.
Scott Witthaus
Senior Editor/Post Production Supervisor
1708 Inc./Editorial
Professor, VCU Brandcenter