The best piece of jazz is a gem! (5)
I believe the answer is:
topaz
'a gem' is the definition.
(topaz is a kind of gem)
'the best piece of jazz' is the wordplay.
'the best' becomes 'top' (top can mean the very best).
'piece of jazz' becomes 'az' (I can't explain this - if you can you should give a lot more credence to this answer).
'top'+'az'='TOPAZ'
'is' is the link.
Can you help me to learn more?
(Other definitions for topaz that I've seen before include "White/colourless mineral used as a gemstone" , "Precious stone - American hummingbird" , "Shade of dark yellow" , "South American hummingbird" , "Type of gemstone" .)