Like o-forms, short comparative adjectives may function as adverbs, not just as predicates. So if you've learned the comparative form of an adjective, you don't have to learn an additional form to make the corresponding adverb.
Thus, just as їсно can mean either 'is clear' (predicate) or 'clearly' (adverb), so can яснље mean either 'is clearer' (predicate) or 'more clearly' (adverb).
Pred. Сегћдня їсно. It is clear today.
  Зђвтра бџдет яснље. Tomorrow it will be clearer.
Adv. Онђ їсно говорќт. She speaks clearly
  Ћн говорќт ещё яснље. He speaks even more clearly.
Listed below are some o-forms with their corresponding short comparatives:
хорошћ лџчше better плћхо хџже worse
мнћго бћльше more мђло мљньше less
бІстро быстрље faster мљдленно мљдленнее more slowly
чђсто чђще oftener рљдко рљже more rarely
грћмко грћмче louder тќхо тќше more quietly, softer