Here's why:
* Phrase: A group of words that functions as a single unit of meaning but does not contain a subject and verb.
* Clause: A group of words that contains a subject and verb and forms a complete thought.
"Long and loud" describes something, but it doesn't have a subject or verb. For example:
* The crowd cheered long and loud. (Here, "long and loud" describes the cheering.)
Let me know if you have any other grammar questions!