H as in Heracles (3)
I believe the answer is:
eta
'h as in heracles' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eta that I've seen before include "giving expected termination of journey" , "Foreign letter" , "Greek letter - Basque nationalist movement" , "When's end of journey predicted" , "Greek letter E; estimated time of arrival (abbr.)" .)