The most common and most likely answer to this crossword clue is the 2 plus 4 plus 5 letter word ON YOUR MARKS
I've seen this crossword clue in The Times
O
N
Y
O
U
R
M
A
R
K
S
More clues leading to the result ON YOUR MARKS
Buddy explains!
The straight part
We know that "How you will be judged in exam/prepare to get going" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case how you will be judged in exam/prepare to get going = on your marks.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.