The most common and most likely answer to this crossword clue is the 8 letter word UNHINGED
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More clues leading to the result UNHINGED
Buddy explains!

The straight part
We know that "Mad" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mad = unhinged.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 UNHINGED mean?
UNHINGED verb- To remove the leaf of a door or a window from its supporting hinges.
- To mentally disturb.
- (usually humorous) Mentally ill or unstable.
- Not furnished with a hinge.
- (of a stamp) Not having ever been mounted using a stamp hinge.