Aristotle's H (3)
I believe the answer is:
eta
'aristotle's h' 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 "European separatists, initially" , "When's end of journey predicted" , "Greek letter - time due (abbr)" , "giving expected termination of journey" , "Basque separatist movement" .)