The most common and most likely answer to this crossword clue is the 6 letter word FORBYE
F
O
R
B
Y
E
Buddy explains!

The straight part
We know that "once past " is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case once past = forbye.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 FORBYE mean?
FORBYE adverb- Past; by; beyond.
- Uncommonly; exceptionally.
- Beyond; past; more than; greater than; over and above; moreover.
- (of time) Past; gone by; over.
- Near; beside; by, close to.
- On one side; out of the way.
- Besides; in addition to; as well as; not to mention.
- With the exception of; not taking into account.