The most common and most likely answer to this crossword clue is the 7 letter word DRUMLIN
I've seen this crossword clue in Independent
D
R
U
M
L
I
N
More clues leading to the result DRUMLIN
Buddy explains!

The straight part
We know that "oval mound" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case oval mound = drumlin.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 DRUMLIN mean?
DRUMLIN noun- An elongated hill or ridge of glacial drift.