The most common and most likely answer to this crossword clue is the 5 letter word MOPES
I've seen this crossword clue in Independent
M
O
P
E
S
More clues leading to the result MOPES
- . Second and third of September, heads West and sulks
- Home counties Brit down under comes over and moons
- Miserly old person, evidently scammed to begin with, is upset
- Sweetheart surrounded by cleaning equipment sulks
- Daughter thrown off vehicles is sad
- Miserable or pining, exhibits sulks initially
- Isn’t happy adapting poems
Buddy explains!

The straight part
We know that "moons" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case moons = mopes.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 MOPES mean?
MOPES verb- To carry oneself in a depressed, lackadaisical manner; to give oneself up to low spirits; to pout, sulk.
- To make spiritless and stupid.