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 tell whether this defines the answer.
'each year in place of independence' is the wordplay.
'each year' becomes 'PA' (abbreviation for per annum).
'in' means one lot of letters goes inside another.
'place of independence' becomes 'ol' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pa' put 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" .)