To Loosen
intransitive verb, godan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means loose, so the verb version is to loosen. This is when something loosens on its own, and not via your actions. You know this because a cow is loosening things, moo (む)...
The reading is the same as the one you learned with the kanji!
そのトンネルのボルトは、何本か緩んできています。
Some of the bolts in the tunnel are coming loose.
彼のジョークにみんなの緊張感が緩んだ。
Everyone's tension loosened at his joke.