The most common and most likely answer to this crossword clue is the 13 letter word OVER-THE-HILL
O
V
E
R
-
T
H
E
-
H
I
L
L
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 = over-the-hill.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.