Ring a mate for something milky (4)
I believe the answer is:
opal
'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" .)