The most common and most likely answer to this crossword clue is the 4 letter word AGED
I've seen this crossword clue in The Daily Telegraph
A
G
E
D
More clues leading to the result AGED
- Old were furious when right’s lost
- Very old French painter returning without son
- Getting on, earning money, but losing something on top
- Characters in carriage driven over the hill?
- Ancient silver beads oddly missing
- Grew old and silver before empty end
- Ran without staff over the hill
- Old French artist, largely upset
- Matured sage Derby shows it
- Getting on with a good editor
Buddy explains!

The straight part
We know that "past it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case past it = aged.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 AGED mean?
AGED verb- To cause to grow old; to impart the characteristics of age to.
- To postpone an action that would extinguish something, as a debt.
- To categorize by age.
- To grow aged; to become old; to show marks of age.
- Old people, collectively.
- Old.
- (chiefly non-US) Having the age of.
- Having undergone the improving effects of time; matured.