Milky white oil’s transformed each year in place of independence (4)
I believe the answer is:
opal
'milky white oil's transformed' is the definition.
I can't judge whether this definition defines the answer.
'each year in place of independence' is the wordplay.
'each year' becomes 'PA' (abbreviation for per annum).
'in' indicates putting letters inside.
'place of independence' becomes 'ol' (I can't explain this - if you can you should give a lot more credence to this answer).
'pa' going into 'ol' is 'OPAL'.
Can you help me to learn more?
(Other definitions for opal that I've seen before include "Milky white gem" , "Iridescent gemstone" , "Milky stuff that's hard" , "Type of gem" , "Delicately coloured gem" .)