Hoax involving Russia's foremost essayist's a gem (9)
I believe the answer is:
cornelian
'a gem' is the definition.
(cornelian is a kind of gem)
'hoax involving russia's foremost essayist's' is the wordplay.
'hoax' becomes 'con'.
'involving' is an insertion indicator.
'foremost' suggests taking the first letters.
'essayist's' becomes 'elian' (to do with Elia, pseudonym of Charles Lamb).
The first letter of 'russias' is 'r'.
'con' going around 'r' is 'corn'.
'corn'+'elian'='CORNELIAN'
(Other definitions for cornelian that I've seen before include "Dull red stone; lone cairn (anagram)" , "Gemstone" , "gem" .)