Worried about Greek character (3)
I believe the answer is:
Here is my best explanation:
'greek character' is the definition.
'worried about' is the subsidiary indication.
'worried' becomes 'ate' (eating is a kind of worrying).
'about' is a reversal indicator.
'ate' reversed gives 'eta'.
(Other definitions for eta that I've seen before include "Member of Greek set a", "letter from Greece?", "character abroad?", "This letter from abroad", "when you can hope to receive it", "European separatists, initially", "giving expected termination of journey", "indicating when flight is due", "Militants" and "Letter of Greek alphabet".)