Spice enthusiast writes about precious stone (6)
I believe the answer is:
nutmeg
'spice' is the definition.
(nutmeg is a kind of spice)
'enthusiast writes about precious stone' is the wordplay.
'enthusiast' becomes 'nut' (nut can mean an enthusiast or fan).
'writes about' is a reversal indicator (I've seen 'written about' mean this).
'precious stone' becomes 'gem' (gem stone).
'gem' back-to-front is 'meg'.
'nut'+'meg'='NUTMEG'
(Other definitions for nutmeg that I've seen before include "Aromatic seed - tropical tree" , "Aromatic seasoning" , "Aromatic seed - met gnu (anag)" , "(In soccer) kick the ball between an opponent's legs" , "Spicy seasoning - hard fruit for little Margaret?" .)