The most common and most likely answer to this crossword clue is the 5 plus 2 plus 5 letter word HEART OF STONE
I've seen this crossword clue in The Guardian
H
E
A
R
T
O
F
S
T
O
N
E
Buddy explains!
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.