"I eat half an apple for breakfast."
Translation:Jem pół jabłka na śniadanie.
maybe they mean second breakfast that has to last from early morning breakfast till Polish "obiad" at 3 pm.
It's close, but you used "połowa" in Nominative, although it should be in Accusative, as needed by "jeść". "Jem połowę jabłka na śniadanie".
With something relatively small, like an apple, the diminutive form "połówka" (Jem połówkę jabłka) sounds better to me, but it doesn't mean that "połowa" is wrong.