Greek character hidden by secret agent (3)

I believe the answer is:
eta
'greek character' is the definition.
(letter from the Greek alphabet)
'hidden by secret agent' is the wordplay.
'hidden' indicates a hidden word.
'ETA' is hidden in the letters of 'by secret agent'.
(Other definitions for eta that I've seen before include "Basque separatist movement" , "giving expected termination of journey" , "Foreign letter" , "indicating when flight is due" , "Arrival roughly indicated" .)