The most common and most likely answer to this crossword clue is the 9 plus 6 letter word IMAGINARY NUMBER
I've seen this crossword clue in The Times
I
M
A
G
I
N
A
R
Y
N
U
M
B
E
R
More clues leading to the result IMAGINARY NUMBER
- Mathematical feature in a made up edition
- Mrs Berry takes a drink – I reckon about….? A non-existent quantity which can be a bit complex
- When small, I may be played by an air-guitarist
- I’m brainy man with urge to work out something in advanced maths
- i invented anaesthetic!
- Curiously Gary Numan and I get honour right after Independent and i
- A bit of maths? Pretend it could send you to sleep!
Buddy explains!
The straight part
We know that "Mathematical feature" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mathematical feature = imaginary number.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.