The most common and most likely answer to this crossword clue is the 4 plus 2 plus 2 letter word LETS IT GO
I've seen this crossword clue in Financial Times
L
E
T
S
I
T
G
O
Buddy explains!
The straight part
We know that "Forgets/it’s what child does to toy when wound up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case forgets/it’s what child does to toy when wound up = lets it go.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.