The most common and most likely answer to this crossword clue is the 5 plus 4 letter word SALAD HERB
I've seen this crossword clue in Independent
S
A
L
A
D
H
E
R
B
More clues leading to the result SALAD HERB
Buddy explains!

The straight part
We know that "rocket?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rocket? = salad herb.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.