The most common and most likely answer to this crossword clue is the 5 letter word MOTTE
I've seen this crossword clue in The Times
M
O
T
T
E
More clues leading to the result MOTTE
Buddy explains!

The straight part
We know that "Ancient mound" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ancient mound = motte.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 MOTTE mean?
MOTTE noun- A raised earth mound, often topped with a wooden or stone structure and surrounded with a ditch.
- A witty remark; a witticism; a bon mot.
- A word or a motto; a device.
- A note or brief strain on a bugle.
- A woman; a wife.
- A prostitute.
- A landlady.
- (chiefly Texas) A copse or small grove of trees, especially live oak or elm.