GPS approximation
I believe the answer is:
eta
'gps approximation' 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 - Basque nationalist movement" , "when you can hope to receive it" , "Militant Basque organisation - Greek letter" , "Member of Greek set a" .)