Crack habit fellow's lost (4)

I believe the answer is:

rock

I'm a little stuck... Click here to teach me more about this clue!

'crack' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I don't see?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for rock that I've seen before include "Lull (baby); 50s pop" , "Perhaps Hudson" , "Large stone - type of music" , "Be unstable; lull (baby)" , "Sway - very big stone" .)

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.