The essence of an epithet (4)
I believe the answer is:
pith
'the essence' is the definition.
(synonyms)
'of an epithet' is the wordplay.
'of' says the answer is hidden in the clue.
'PITH' can be found hidden inside 'epithet'.
(Other definitions for pith that I've seen before include "Tissue inside the rind of citrus fruits" , "Fibrous part of fruit - essence" , "Core - essence" , "White tissue under the rind of citrus fruit" , "Central part" .)