The most common and most likely answer to this crossword clue is the 4 plus 6 letter word GOOD HEALTH
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More clues leading to the result GOOD HEALTH
Buddy explains!
The straight part
We know that "For which we eat an apple a day/toast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case for which we eat an apple a day/toast = good health.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.