"Yes, an apple."
Read the notes on the skill "Accusative":
"THE ACCUSATIVE ENDING: -N
In Esperanto, a special ending is required to show that a noun is the direct object of a statement. Let's look at the English sentence "A man kisses a woman." To show that a particular noun is a direct object (receives the action), always add an -n to the end of that noun or pronoun. Thus, the most usual translation of this sentence would be Viro kisas virinon. Another way of thinking of this is to ask who or what is receiving the action, in this case the woman virinon."
So apple is pomo. But if you say "I eat an apple", you would say "Mi manĝas pomon", with -N at the end, because it's the direct oject of that sentence.
But "Yes, an apple" is a complete sentence? I don't think the accusative has to do with things that is out of the sentence.
(There is the tricky case of some expressions where some part of a sentence are regarded as being omitted, as in "good night" actiually meaning ...whatever the example was, previously.. :) I'm a newbie, so take it with a grain of salt. But those cases are about something in the same sentence that are omitted, not something out of the sentence.)