"Clearly" is already an adverb in its superlative form, meaning "very clearly" or "most clearly".
Here are some ways to express a higher degree of clarity:
* Very clearly: This is the most common way to increase the intensity.
* Crystal clear: This emphasizes the lack of ambiguity.
* Unmistakably clear: This emphasizes the certainty.
* Absolutely clear: This emphasizes the completeness of understanding.
Instead of looking for a single superlative word, consider using phrases that better express the level of clarity you wish to convey.