The most common and most likely answer to this crossword clue is the 6 letter word MENTAL
I've seen this crossword clue in Financial Times
M
E
N
T
A
L
More clues leading to the result MENTAL
- Intellectual fellows converse endlessly
- Fellows curtailed discussion of the mind
- Crazy fellows chatter endlessly
- Fellows chat endlessly — it’s all in the mind
- Crazy being irritable after losing paint
- Crazy lament?
- Intellectual pieces detailed story
- Food given outside theatre off trolley
- Crazy man let loose
- Intellectual restricted by government always
Buddy explains!

The straight part
We know that "Crazy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case crazy = mental.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 MENTAL mean?
MENTAL adjective- Of or relating to the mind or an intellectual process.
- Insane, mad, crazy.
- Enjoyable or fun, especially in a frenetic way.
- A plate or scale covering the mentum or chin of a fish or reptile.
- Of or relating to the chin or median part of the lower jaw, genial.
- Of or relating to the chin-like or lip-like structure.