The most common and most likely answer to this crossword clue is the 6 letter word RARITY
I've seen this crossword clue in The Guardian
R
A
R
I
T
Y
More clues leading to the result RARITY
- I tarry in resort: that’s not seen often
- Beam about runs scored: it's an uncommon thing
- Gunmen on line claiming it’s a freak
- Republican with desperate air and vacuous Trumpery that’s seldom met
- In the light, dealer’s ultimate object of quest!
- Gunners side taking international trophy borders on strangeness
- It’s unusual for the Queen’s Trust to exclude the Church
- It’s not common for equivalence to replace power with resistance
- Find hint about slowing in music
- It’s hard to find it within artillery lines
Buddy explains!

The straight part
We know that "It’s unusual" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s unusual = rarity.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 RARITY mean?
RARITY noun- A measure of the scarcity of an object.
- (of a gas) Thinness; the property of having low density
- A rare object.