The most common and most likely answer to this crossword clue is the 11 letter word METAMORPHIC
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "of some rocks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of some rocks = metamorphic.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 METAMORPHIC mean?
METAMORPHIC noun- A rock that has been changed from its original form by subjection to heat and/or pressure.
- Characterised by or exhibiting a change in form or character.
- Pertaining to metamorphism; having been structurally altered as a result of, or resulting from, exposure to intense heat and/or pressure (at the contact zone between colliding plates, for example).
- Pertaining to metamorphosis.
- Of or relating to the Metamorphic Technique, a form of massage influenced by reflexology.