Letter after epsilon (4)
I believe the answer is:
zeta
'letter after epsilon' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for zeta that I've seen before include "Spartan character" , "6th Greek letter" , "Greek letter Z" , "Greek letter (for Catherine Jones?)" , "Letter to the Corinthians" .)