Fragment, One Of A Pair, Incomplete
prefix
Something that is one-sided has only one side and not the whole. That's just one of a pair or a fragment of the whole. Stick this onto another word and you have one of a pair or an "incomplete_____."
かた
The reading is the same as the one you learned with the kanji.
片目だけ視力が悪いんです。
I have a hard time seeing out of one of my eyes.