Greek letter is omitted, taken away initially (4)
I believe the answer is:
iota
'greek letter' is the definition.
(letter from the Greek alphabet)
'is omitted taken away initially' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'is omitted taken away' is 'IOTA'.
(Other definitions for iota that I've seen before include "The smallest possible amount, or the Greek i" , "Ninth letter of Greek alphabet, small amount" , "Smidgeon" , "Trace" , "Greek letter of little value" .)