The most common and most likely answer to this crossword clue is the 3 letter word FEY
F
E
Y
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The straight part
We know that "slightly mad" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case slightly mad = fey.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 FEY mean?
FEY adjective- About to die; doomed; on the verge of sudden or violent death.
- Dying; dead.
- Possessing second sight, clairvoyance, or clairaudience.
- Overrefined, affected.
- Strange or otherworldly.
- Spellbound.
- Fairy folk collectively.
- Magical or fairylike.