Jazz fans look upon chrysoberyl (4-3)
I believe the answer is:
'chrysoberyl' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I don't see?
'jazz fans look' is the wordplay.
'jazz fans' becomes 'cats' (I've seen this in another clue).
'look' becomes 'eye' (eyeing is a kind of looking).
'upon' is the link.
Can you help me to learn more?
(Other definitions for cats-eye that I've seen before include "Light-reflecting stud" , "Reflective stud" , "Reflector used in roads" , "Small glass device in a road surface" , "Gem; road reflector" .)