Duck, chum - here comes a stone! (4)

I believe the answer is:

opal

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

'stone' is the definition.
(gemstone)

'duck chum' is the wordplay.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'chum' becomes 'pal' (both can mean a friend).
'o'+'pal'='OPAL'

'here comes a' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for opal that I've seen before include "Precious stuff" , "Rock" , "Form of hydrated silica, often used as a gemstone" , "precious jewel" , "raw material" .)

I've seen this clue in the Evening Standard.

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.