Letter after zeta (3)
I believe the answer is:
eta
'letter after zeta' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eta that I've seen before include "Militants" , "Greek letter - time due (abbr)" , "When you'd expect to see" , "when it might reach us?" , "Basque separatist movement" .)