The most common and most likely answer to this crossword clue is the 6 letter word GABBRO
G
A
B
B
R
O
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The straight part
We know that "Rock" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rock = gabbro.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 GABBRO mean?
GABBRO noun- Originally, a kind of serpentine; now generally a coarsely crystalline, igneous rock consisting of lamellar pyroxene and labradorite.