The most common and most likely answer to this crossword clue is the 3 plus 3 plus 2 plus 5 letter word PUT OUT TO GRASS
I've seen this crossword clue in Independent
P
U
T
O
U
T
T
O
G
R
A
S
S
More clues leading to the result PUT OUT TO GRASS
Buddy explains!
The straight part
We know that "Treated as past it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case treated as past it = put out to grass.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.