The most common and most likely answer to this crossword clue is the 5 letter word DEFAT
I've seen this crossword clue in The Times
D
E
F
A
T
More clues leading to the result DEFAT
Buddy explains!

The straight part
We know that "prepare for Jack to?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case prepare for jack to? = defat.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 DEFAT mean?
DEFAT verb- To remove fat from a material, especially by the use of solvents