The most common and most likely answer to this crossword clue is the 4 plus 3 plus 2 letter word COME OFF IT
I've seen this crossword clue in Financial Times
C
O
M
E
O
F
F
I
T
More clues leading to the result COME OFF IT
- Get down from there? You’re kidding!
- Succeed the first time? Don’t be ridiculous!
- Result from match that’s most unlikely
- How to stop sex addiction? Don’t make me laugh!
- Desist from computers and the like? Don’t be ridiculous!
- Advice to addict? Don’t be absurd!
- What not to do on a high wire? You’re kidding!
- Succeed with Italian — you must be joking!
- Prove successful with modern technology? Don’t be daft!
- Italian must go after fall? Don’t be ridiculous!
Buddy explains!

The straight part
We know that "Get down from there?/You’re kidding!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case get down from there?/you’re kidding! = come off it.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.