Carved from tufa stone, immoveable (4)
I believe the answer is:
fast
'immoveable' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for fast that I've seen before include "Don't scoff" , "Rapid - diet" , "Quick - abstain from food" , "Secure; speedy" , "Starve; quick" .)