晟. Is this kanji used in compounds? I saw it in the dictionary but there isn’t much info on it except that it is an old JLPT 1 kanji, means “clear”, and has kun reading “akiraka”. Is this a common kanji?
This is indeed a rare kanji, but it is on the Kanji Kentei list, Level 1. It is listed in the standard dictionaries, but there don't seem to be any compounds containing this character which are in regular use. It may occur in proper nouns like place names or people names. There is some info on it here. But this is not a character which you are likely to encounter unless you are studying for a high-level Japanese test.