Essentially Amazon Prime's first gem (5)
I believe the answer is:
topaz
'gem' is the definition.
(I know that topaz is a type of transparent gem)
'essentially amazon prime's first' is the wordplay.
'essentially' says to take the centre.
'prime' becomes 'top' (both can mean excellent).
'first' is a charade indicator (letters next to each other).
The middle letters of 'amazon' are 'az'.
'az' after 'top' is 'TOPAZ'.
(Other definitions for topaz that I've seen before include "Precious stone - American hummingbird" , "Silicate gemstone" , "Semi-precious stone of various colours, especially yellow" , "Shade of dark yellow" , "Precious stone - dark yellow colour" .)