Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You haven't learned this reading yet so here's a mnemonic to help you:
You have to narrow the width of something. The only way you can do that is to sever (せば) off each end. Maybe you're trying to get a sofa up a tight stairway. The only way to do that is by severing off each arm to narrow it.
Can you narrow the width a little more here?