The most common and most likely answer to this crossword clue is the 8 letter word ROCKROSE
I've seen this crossword clue in The Times
R
O
C
K
R
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S
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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 = rockrose.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 ROCKROSE mean?
ROCKROSE noun- Plants in the family Cistaceae, the "rock rose family" or sunroses.
- Pavonia lasiopetala (Malvaceae), Texas swampmallow.
- Phemeranthus spp. (Montiaceae), flameflower.
- Portulaca grandiflora (Portulacaceae) moss-rose purslane.