The most common and most likely answer to this crossword clue is the 4 letter word MONS
I've seen this crossword clue in The Guardian
M
O
N
S
More clues leading to the result MONS
Buddy explains!
The straight part
We know that "Battle in Belgium on Sunday" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case battle in belgium on sunday = mons.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 MONS mean?
MONS noun- One of the fleshy areas at the base of the fingers; a mount.
- The pubic mound or mons pubis. In human anatomy or in mammals in general, the mons pubis (Latin for "pubic mound"), also known as the mons veneris (Latin, mound of Venus) or simply the mons, is the adipose tissue lying above the pubic bone of adult females, anterior to the pubic symphysis. The mons pubis forms the anterior portion of the vulva.
- A mountain or extinct volcano on a planet or a moon.