The most common and most likely answer to this crossword clue is the 9 letter word GREYBEARD
I've seen this crossword clue in The Daily Telegraph
G
R
E
Y
B
E
A
R
D
More clues leading to the result GREYBEARD
Buddy explains!

The straight part
We know that "Old man" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old man = greybeard.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 GREYBEARD mean?
GREYBEARD noun- An old man.
- Any of the members of a group who have been there the longest, often implying experience.
- A coarse earthenware vessel for holding liquor; a bellarmine.