Greek letter gobbled up (3)
I believe the answer is:
eta
'greek letter' is the definition.
(letter from the Greek alphabet)
'gobbled up' is the wordplay.
'gobbled' becomes 'ate' (to gobble is to eat food).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'ate' backwards is 'ETA'.
(Other definitions for eta that I've seen before include "character abroad?" , "Militant Basque organisation - Greek letter" , "when it might reach us?" , "Seventh Greek letter" , "Letter of Greek alphabet" .)