The most common and most likely answer to this crossword clue is the 4 plus 1 letter word ONCE -
O
N
C
E
-
Buddy explains!

The straight part
We know that "past" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case past = once -.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 ONCE - mean?
ONCE - adverb- (frequency) One and only one time.
- (temporal location) Formerly; during some period in the past.
- Multiplied by one: indicating that a number is multiplied by one.
- As soon as.
- At a future time.
- As soon as; when; after.