Child beginning to suck kind of rock (5)
I believe the answer is:
sprog
'child' is the definition.
(informal term for a child)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sprog that I've seen before include "Baby (slang)" , "little one" , "Infant" , "Child; recruit (mil. slang)" , "Kid" .)