The most common and most likely answer to this crossword clue is the 9 letter word DIFFERENT
I've seen this crossword clue in Financial Times
D
I
F
F
E
R
E
N
T
More clues leading to the result DIFFERENT
- New doctor fired, entertaining fine hospital department
- Rare Fender, if electric, beginning to tempt
- Unlike poor friend, left with no end of approval
- Unusual gear? Just three-quarters of it
- Separate hollow covers providing iron earth’s core
- Not like to be fed if “mixed raw starter” extends to chicken portion and rat’s tail
- Unusual fender — fit in a new way
- Not the same if fellow is in rented out accommodation
Buddy explains!

The straight part
We know that "Rare" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rare = different.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 DIFFERENT mean?
DIFFERENT noun- The different ideal.
- Not the same; exhibiting a difference.
- Various, assorted, diverse.
- Distinct, separate; used for emphasis after numbers and other determiners of quantity.
- Unlike most others; unusual.
- Differently.