Coloured stone that’s become white when removed from casing (4)
I believe the answer is:
opal
'coloured stone' is the definition.
'opal' can be an answer for 'stone' (I've seen this before). I'm not sure about the 'coloured' bit.
'become white when removed from casing' is the wordplay.
I cannot really understand how this works, but
'from' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
The remaining letters 'pal' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for opal that I've seen before include "Milky iridescent stone" , "Translucent gem" , "One in a ring perhaps" , "often semi-transparent" , "Milky-white gemstone with flashes of colour" .)