The most common and most likely answer to this crossword clue is the 4 letter word UPON
I've seen this crossword clue in The Daily Telegraph
U
P
O
N
More clues leading to the result UPON
- Company abandoned voucher: it’s over!
- In contact with contributor to group online
- Excited about getting over
- Above suspicion, but no saint, not special, not here in France
- Just above being well-informed
- Supported by leaders of union, picket office next
- Supported by [amazingly] cheerful working
- A preposition taking an adverb is still a superior preposition
- Superior to a bad pun about love
- Following bend on river, source of Nile
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 UPON mean?
UPON adverb- Being the target of an action.
- Physically above and in contact with.
- Physically directly supported by.
- Being followed by another so as to form a series.
- At (a prescribed point in time).
- On.