The most common and most likely answer to this crossword clue is the 4 plus 3 plus 5 letter word MISS THE POINT
I've seen this crossword clue in The Guardian
M
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S
S
T
H
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P
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N
T
More clues leading to the result MISS THE POINT
Buddy explains!
The straight part
We know that "Fail to understand decimals correctly?/Fail to understand decimals correctly?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fail to understand decimals correctly?/fail to understand decimals correctly? = miss the point.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.