Here's why:
* It modifies a verb, adjective, or another adverb. For example:
* "He clearly explained the problem." (modifies the verb "explained")
* "She had a clearly defined plan." (modifies the adjective "defined")
* "They spoke clearly and confidently." (modifies the adverb "confidently")
Let me know if you have any other words you'd like to identify the part of speech for!