The most common and most likely answer to this crossword clue is the 4 letter word THEN
I've seen this crossword clue in Financial Times
T
H
E
N
More clues leading to the result THEN
Buddy explains!

The straight part
We know that "Once upon a time" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case once upon a time = then.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 THEN mean?
THEN conjunction- Used in comparisons, to introduce the basis of comparison.
- (obsolete outside dialectal, usually used with for) Because; for.
- That time
- Being so at that time.
- (temporal location) At that time.
- (temporal location) Soon afterward.
- (sequence) Next in order; in addition.
- In that case.
- (sequence) At the same time; on the other hand.
- (affirmation) Used to contradict an assertion.