I just tried it in Multitrack by highlighting a portion of a track to see if the effect would apply just to the highlighted portion. Didn't work. But here's something that I know *will* work. You can highlight the portion of the track that you want to effect, then clone/copy that highlighted portion to a new empty track. Then run the effect on the cloned portion and, using the volume envelope, mute the portion of the original track that you don't want to hear.