Grub originally revealed in rock (5)
I believe the answer is:
larva
'grub' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for larva that I've seen before include "shows form of immaturity" , "a developmental stage" , "Maggot" , "Insect's feeding stage" , "Worm-like insect before metamorphosis" .)