The most common and most likely answer to this crossword clue is the 13 letter word STONY-HEARTED
I've seen this crossword clue in The Guardian
S
T
O
N
Y
-
H
E
A
R
T
E
D
More clues leading to the result STONY-HEARTED
Buddy explains!

The straight part
We know that "Uncaring" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case uncaring = stony-hearted.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 STONY-HEARTED mean?
STONY-HEARTED adjective- Cruel, compassionless, discompassionate, as if having a heart made of stone.