The most common and most likely answer to this crossword clue is the 6 letter word RAVENS
I've seen this crossword clue in The Guardian
R
A
V
E
N
S
Buddy explains!

The straight part
We know that "Unkindness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unkindness = ravens.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 RAVENS mean?
RAVENS noun- Any of several, generally large and lustrous black species of birds in the genus Corvus, especially the common raven, Corvus corax.
- A jet-black colour.
- Rapine; rapacity.
- Prey; plunder; food obtained by violence.