Crossword Genius

Ring a mate for something milky (4)

Ross

I believe the answer is:

opal

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

'something milky' is the definition.
I can't tell whether this definition defines the answer.

'ring a mate' is the wordplay.
'ring' becomes 'O' (letter resembling a ring).
'a mate' becomes 'pal' (I've seen this before).
'o'+'pal'='OPAL'

'for' acts as a link.

Can you help me to learn more?

(Other definitions for opal that I've seen before include "Milky-white gemstone with flashes of colour" , "often semi-transparent" , "what may be set in 29 [DIAL]" , "Iridescent quartz-like stone" , "One's stony" .)

I've seen this clue in the Evening Standard.
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