This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. This word is actually two words put together, though. 亡く means "be deceased." なる means "to become." So put that all together and you have to pass away or to become deceased. It's basically a polite way to say something died (死ぬ).

To learn to read this kanji you have to remember the な part of 亡く, though our mnemonic will use the whole thing.

If you're about to become deceased, you are probably knocking (なく) on death's door.