The most common and most likely answer to this crossword clue is the 5 letter word ONCER
I've seen this crossword clue in The Guardian
O
N
C
E
R
Buddy explains!

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 ONCER mean?
ONCER noun- A one-pound note.
- A person who does something once.
- A single sexual encounter between two individuals, a one-night stand.
- A person with whom one shares a single sexual encounter, a one-night stand.