Letter two before iota (3)
I believe the answer is:
eta
'letter two before iota' 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 "Letter from Doris" , "when you can hope to receive it" , "when it might reach us?" , "giving expected termination of journey" , "indicating when flight is due" .)