Brother escaping from erupting subregion describing rocks (7)
I believe the answer is:
igneous
'brother' is the definition.
Although both the answer and definition are adjectives, I can't understand how they can define each other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for igneous that I've seen before include "Of fire, fiery" , "(Of rock) solidified from lava or magma" , "Category of rock, being of solidified molten magma" , "Maybe fiery" , "Of fire, like rocks formed from molten material" .)