Essayist getting attorney to reveal Greek letter (6)
I believe the answer is:
lambda
'greek letter' is the definition.
(letter from the Greek alphabet)
'essayist getting attorney' is the wordplay.
'essayist' becomes 'lamb' (I've seen this in another clue).
'getting' says to put letters next to each other.
'attorney' becomes 'DA' (District Attorney).
'lamb'+'da'='LAMBDA'
'to reveal' is the link.
(Other definitions for lambda that I've seen before include "Teasing entertainer" , "Greek letter - am bald (anag)" , "The 11th letter of the Greek alphabet" , "Greek letter L" , "character from Greece" .)