The most common and most likely answer to this crossword clue is the 4 letter word TYPO
I've seen this crossword clue in The Times
T
Y
P
O
More clues leading to the result TYPO
- Printing error in naughty postcard
- Printing error: treaty gutted and work set back
- Finger problem exhibited by party pooper
- Mistake, kind that has one letter wrong?
- Error that you print out at first
- Mistake turning up in copy, twice
- Wrong character, perhaps, to enter party politics
- Key error admitted by party-pooper
- Key mistake admitted by party-pooper
- Some blood not English: a simple mistake?
Buddy explains!

The straight part
We know that "Key error" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case key error = typo.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 TYPO mean?
TYPO noun- A typographical error.
- To make a typographical error.
- A compositor; a typographer.