Being milky means nothing, mate (4)
I believe the answer is:
opal
'being milky means nothing mate' is the definition.
I can't judge whether this definition defines the answer.
'nothing mate' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'mate' becomes 'pal' (I've seen this before).
'o'+'pal'='OPAL'
Can you help me to learn more?
(Other definitions for opal that I've seen before include "Precious stone" , "Milky-white gemstone with flashes of colour" , "Mineral, usually iridescent" , "Milk-coloured gemstone" , "Semi-precious stone of milky or bluish colour" .)