clear, lucid
澄む（すむ）、澄ます（すます）、清澄（せいちょう）
to be clear, to look serious, clear
On-reading (チョウ) is used in compounds; kun-reading (す(む)) is used standalone or with okurigana. Appears in many compounds — memorize the key ones rather than the kanji in isolation. This kanji forms verbs/adjectives with okurigana. Practice both the word form and compound forms.
From the JLP Quiz question bank — the word in the form it is actually tested in.
「研ぎ澄ます」の読み方はどれか。
Answer: とぎすます
