The most common and most likely answer to this crossword clue is the 5 letter word CHOOK
C
H
O
O
K
Buddy explains!

The straight part
We know that "One hatching" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one hatching = chook.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 CHOOK mean?
CHOOK noun- A chicken, especially a hen.
- A cooked chicken; a chicken dressed for cooking.
- A fool.
- A call made to chickens.