Answer: The participial phrase in the sentence is "practicing the clarinet" and the word modified is "Melissa".
Step-by-step explanation: A participial phrase is a phrase that begins with a past participle or present participle which is followed by an object and, in some cases, modifiers. Moreover, a participial phrase always acts as an adjective. In the sentence "I heard Melissa practicing the clarinet", "practicing the clarinet" is a participial phrase because it is made up of a present participle ("practicing") and an object ("the clarinet"). Furthermore, this participial phrase is modifying "Melissa", which is the direct object in the sentence. In other words, "practicing the clarinet" is describing and providing additional information about "Melissa".