The most common and most likely answer to this crossword clue is the 10 letter word CURMUDGEON
I've seen this crossword clue in The Guardian
C
U
R
M
U
D
G
E
O
N
More clues leading to the result CURMUDGEON
Buddy explains!

The straight part
We know that "Cantankerous old person" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cantankerous old person = curmudgeon.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 CURMUDGEON mean?
CURMUDGEON noun- A miser.
- An ill-tempered person full of stubborn ideas or opinions.