Greek character is missing from this Greek letter (5)
I believe the answer is:
theta
'greek character' is the definition.
(letter from the Greek alphabet)
'is missing from this greek letter' is the wordplay.
'missing from' is a deletion indicator.
'greek letter' becomes 'eta' (letter from the Greek alphabet).
'this' with 'is' taken out is 'th'.
'th'+'eta'='THETA'
(Other definitions for theta that I've seen before include "Character from overseas" , "classic character?" , "Letter before iota" , "Letter from Greece" , "Eighth letter of Greek alphabet" .)