The most common and most likely answer to this crossword clue is the 5 letter word MAGMA
I've seen this crossword clue in The Times
M
A
G
M
A
More clues leading to the result MAGMA
- “Flower of Pompeii” publication supported by postgraduate
- Periodical by graduate is hot stuff!
- Papa's partner embracing good, hot rock
- Molten rock mother observed around foot of berg
- Mum swallows good hot stuff
- Mother grasps baking at last — it's very hot under the crust
- Molten rock mother found crossing top of gorge
- Molten material‘s reduced periodical state
- In retrospect a film studio entertains adult with hot material
- Publication with nude imam is hot stuff!
Buddy explains!

The straight part
We know that "hot rock" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hot rock = magma.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 MAGMA mean?
MAGMA noun- The molten matter within the earth, the source of the material of lava flows, dikes of eruptive rocks, etc.
- A basic algebraic structure consisting of a set equipped with a single binary operation.
- Any soft doughy mass.
- The residuum after expressing the juice from fruits.