Corner, Nook
noun
A one sided corner is still a corner, just a really small one.
かたすみ
The readings for this word are both kun'yomi readings. You learned them when you learned the kanji, so you should be able to read this on your own.
そのことが心の片隅にずっと引っかかっていたんです。
That had always been stuck in the corner of my mind.