The most common and most likely answer to this crossword clue is the 7 letter word MISTOOK
I've seen this crossword clue in The Daily Telegraph
M
I
S
T
O
O
K
Buddy explains!

The straight part
We know that "Was wrong" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case was wrong = mistook.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 MISTOOK mean?
MISTOOK verb- To understand wrongly, taking one thing or person for another.
- To misunderstand (someone).
- To commit an unintentional error; to do or think something wrong.
- To take or choose wrongly.