The correct phrase is ‘more clearly.’ In English, when we want to compare actions or states and the adverb in question has two or more syllables, we typically form the comparative by adding ‘more’ before the adverb.
‘Clearly’ is an adverb with three syllables, so we say ‘more clearly’ to express a comparative degree, as in ‘She explained the concept more clearly than before.’
On the other hand, ‘clearlier’ is not a standard English word. While in some cases shorter adverbs can take the ‘-er’ suffix (like ‘fast’ becoming ‘faster’), in the case of longer adverbs like ‘clearly,’ we adhere to the rule of using ‘more.’