Greek letter, originally a vowel but later a consonant (3)
I believe the answer is:
eta
'greek letter' is the definition.
(letter from the Greek alphabet)
'originally a vowel but later a consonant' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'vowel' could be 'e' and 'e' is located in the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for eta that I've seen before include "When you'd expect to see" , "letter from Greece?" , "Militants" , "Greek letter - time due (abbr)" , "when you can hope to receive it" .)