I don't know what edition or imprint you have of Merriam-Webster's Dictionary, but mine, the digital Merriam-Webster's 11th Collegiate Dictionary, has separate entries for "cleverer" and "cleverest" to recognize their existence in the English lexicon. These two words, however, are simply intensifiers of the adjective "clever," and the dictionary assumes they are already understood as such by the reader. As a matter of style by the dictionary, therefore, these intensifiers no longer get separate entries with definitions nor are mentioned in the definition of the source word itself.