The most common and most likely answer to this crossword clue is the 3 letter word VEG
I've seen this crossword clue in Independent
V
E
G
More clues leading to the result VEG
Buddy explains!

The straight part
We know that "five a day ideally?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case five a day ideally? = veg.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.
What does VEG mean?
VEG noun- Vegetable.
- To vegetate; to engage in complete inactivity; to rest
- Vegetarian
- A unit of subjective weight, equivalent to the perceived weight of lifting 100 grams.