The most common and most likely answer to this crossword clue is the 13 letter word SUPERANNUATED
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More clues leading to the result SUPERANNUATED
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 = superannuated.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 SUPERANNUATED mean?
SUPERANNUATED verb- To retire or put out of use due to age.
- To show to be obsolete due to age.
- To retire due to age.
- To become obsolete or antiquated.
- To give a pension to, on account of old age or other infirmity; to cause to retire from service on a pension.
- Obsolete, antiquated.
- Retired or discarded due to age.