Koichi, put your toys away! If you don’t I’ll put you in the dark, narrow closet.
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of narrow? It's narrow.
The reading is the same as the kanji, so you're good to go here. It may be a narrow way of thinking about this kanji, but it will help you out the most.