The most common and most likely answer to this crossword clue is the 8 letter word FORENOON
F
O
R
E
N
O
O
N
Buddy explains!

The straight part
We know that "it’s up to PM" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s up to pm = forenoon.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 FORENOON mean?
FORENOON noun- The part of the day from dawn to noon.
- The part of the day between midnight and noon.
- The early part of anything.
- The first alcoholic drink of the day; a morning draught.