The most common and most likely answer to this crossword clue is the 8 letter word JUMPSUIT
I've seen this crossword clue in The Guardian
J
U
M
P
S
U
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T
More clues leading to the result JUMPSUIT
Buddy explains!
The straight part
We know that "Onesie" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case onesie = jumpsuit.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 JUMPSUIT mean?
JUMPSUIT noun- A one-piece item of clothing originally used by parachutists
- A similar item of clothing used for outdoor sports such as skiing