This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means narrow, so the verb version is to narrow.

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 gap between yourself and your enemy so that you can sabotage (せば) them. You're trying to get through this already narrow air vent, so you can sabotage all of their plans! (But because your windpipe is so narrow in this air vent, it comes out more like sebatage.)