Unknown projected delivery date for letter from abroad (4)

I believe the answer is:

zeta

I'm a little stuck... Click here to teach me more about this clue!

'letter from abroad' is the definition.
(letter of the Greek alphabet)

'unknown projected delivery date' is the wordplay.
'unknown' becomes 'z' (algebra).
'projected delivery date' becomes 'eta' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'z'+'eta'='ZETA'

'for' acts as a link.

Can you help me to learn more?

(Other definitions for zeta that I've seen before include "Letter to the Corinthians" , "Welsh actress, Catherine __-Jones, b. 1969" , "6th Greek letter" , "Sixth letter of the Greek alphabet" , "Greek letter Z" .)

I've seen this clue in The Telegraph.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.