clear up has 20 distinct synonyms spread across 5 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is overcast.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
finish a task completely
“I finally got through this homework assignment”
make free from confusion or ambiguity; make clear
“Could you clarify these remarks?”
Synonyms, antonyms and the broader and narrower relations for clear up come from Princeton University’s WordNet. Full sources and licences.