Letter and when it should get here (3)
I believe the answer is:
eta
'letter' is the definition.
(eta is a kind of letter)
'when it should get here' is the wordplay.
I cannot really understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
The remaining letters 'ea' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'and' is the link.
Can you help me to learn more?
(Other definitions for eta that I've seen before include "When you'd expect to see" , "when it might reach us?" , "Greek character?" , "indicating when flight is due" , "Foreign letter" .)