One’s skill in quiz crystal clear (6)
I believe the answer is:
quartz
'quiz crystal clear' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'one's skill' is the wordplay.
I cannot quite understand how this works, but
'skill' could be 'art' (both can mean a talent) and 'art' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for quartz that I've seen before include "Crystalline mineral" , "Colourless/white mineral" , "Rock-forming mineral" , "Hard glossy mineral present in many rocks" , "Mineral used in modern timepieces" .)