GPS guess (3)
I believe the answer is:
eta
'gps guess' 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 "Greek letter - time due (abbr)" , "Greek letter - Basque nationalist movement" , "European separatists, initially" , "Member of Greek set a" , "Basque separatist movement" .)