Seventh Greek letter (3)
I believe the answer is:
eta
'seventh greek letter' 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 "Something like 10 [RHO]" , "European separatists, initially" , "When's end of journey predicted" , "Militant Basque organisation - Greek letter" , "Greek letter E; estimated time of arrival (abbr.)" .)