Greek character is absent in this Greek letter (5)
I believe the answer is:
theta
'greek character' is the definition.
(letter from the Greek alphabet)
'is absent in this greek letter' is the wordplay.
'absent in' suggests deleting specific letters.
'greek letter' becomes 'eta' (letter from the Greek alphabet).
'this' with 'is' taken away is 'th'.
'th'+'eta'='THETA'
(Other definitions for theta that I've seen before include "classic character?" , "Letter before iota" , "Foreign letter" , "Eighth letter of Greek alphabet" , "Foreign character" .)