It's in the jeweller's window your mates are about to break (8)
I believe the answer is:
amethyst
'it's in the jeweller's window your' is the definition.
I can't tell whether this defines the answer.
'mates are about to break' is the wordplay.
'are about' is an anagram indicator.
'to' becomes 'thy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'break' is an insertion indicator (inserted letters break into the word).
'mates' with letters rearranged gives 'amest'.
'amest' going around 'thy' is 'AMETHYST'.
Can you help me to learn more?
(Other definitions for amethyst that I've seen before include "Type of quartz" , "Variety of quartz" , "Transparent bluish quartz" , "Precious stone of purple quartz" , "Purple quartz used in jewellery" .)