Cryptic crossword clue
a     This is impossible without oxygen 2-2

The most common and most likely answer to this crossword clue is the 5 letter word NO-NO

N
O
-
N
O

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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 NO-NO mean?

NO-NO noun
  1. Something that is forbidden, prohibited, discouraged or taboo.
  2. A no-hitter; a game in which no batter on one of the teams got a hit.

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