Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
You have something that's really long. It's this crazy hair growing out of the tip of your nose. You don't really care either way if it stays or goes, but you're too lazy to cut it off. Your parents, though, keep trying to nag (なが) you to trim it. Imagine this process happening and be irritated about it. What naggers.
This movie was long.
That lady with long hair standing by the door is the one in charge.
I'm thinking about breaking my own legs to avoid going long distance running.