Greek character cutting top off cheese (3)
I believe the answer is:
eta
'greek character' is the definition.
(letter from the Greek alphabet)
'cutting top off cheese' is the wordplay.
'cutting top off' means to remove the first letter.
'cheese' becomes 'feta' (type of cheese).
'feta' with its first letter taken away is 'ETA'.
(Other definitions for eta that I've seen before include "Arrival roughly indicated" , "Greek letter - time due (abbr)" , "character abroad?" , "Member of Greek set a" , "Militant Basque organisation - Greek letter" .)