The most common and most likely answer to this crossword clue is the 6 letter word PUPATE
I've seen this crossword clue in Independent
P
U
P
A
T
E
More clues leading to the result PUPATE
- Young animal was taking food to effect a transformation
- In revolt, blocking head’s move to new form
- To reach an intermediate stage, young pet was tucking in
- Dog’s breakfast was to become temporarily dormant
- Youngster worried to change form
- To develop biologically, little animal took food
- To reach near-perfect stage of development, young animal took food
Buddy explains!

The straight part
We know that "to change form" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to change form = pupate.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 PUPATE mean?
PUPATE verb- To become a pupa.