The most common and most likely answer to this crossword clue is the 5 letter word HOARY
I've seen this crossword clue in The Guardian
H
O
A
R
Y
Buddy explains!
The straight part
We know that "ancient" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ancient = hoary.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 HOARY mean?
HOARY adjective- White, whitish, or greyish-white.
- White or grey with age.
- Of a pale silvery grey.
- Covered with short, dense, greyish white hairs; canescent.
- Remote in time past.
- Moldy; mossy; musty.
- Old or old-fashioned.