The most common and most likely answer to this crossword clue is the 6 letter word NOVICE
I've seen this crossword clue in Financial Times
N
O
V
I
C
E
More clues leading to the result NOVICE
Buddy explains!

The straight part
We know that "rookie" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rookie = novice.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 NOVICE mean?
NOVICE noun- A beginner; one who is not very familiar or experienced in a particular subject.
- A new member of a religious order accepted on a conditional basis, prior to confirmation.