The most common and most likely answer to this crossword clue is the 5 letter word PUPAE
I've seen this crossword clue in The Daily Telegraph
P
U
P
A
E
More clues leading to the result PUPAE
Buddy explains!
The straight part
We know that "Future monarchs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case future monarchs = pupae.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 PUPAE mean?
PUPAE noun- An insect in the development stage between larva and adult.