The most common and most likely answer to this crossword clue is the 9 letter word SPADEWORK
I've seen this crossword clue in The Guardian
S
P
A
D
E
W
O
R
K
Buddy explains!

The straight part
We know that "Prep" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case prep = spadework.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 SPADEWORK mean?
SPADEWORK noun- Work done by digging with a spade.
- Work done in preparation for something else.