The most common and most likely answer to this crossword clue is the 9 letter word ROSEMARY*
I've seen this crossword clue in The Guardian
R
O
S
E
M
A
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Buddy explains!
The straight part
We know that "Shrub" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shrub = rosemary*.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 ROSEMARY* mean?
ROSEMARY* noun- A shrub, Rosmarinus officinalis, that originates from Europe and Asia Minor and produces a fragrant herb used in cooking and perfumes.