Teaching alphabet including old Greek character (4)
I believe the answer is:
iota
'greek character' is the definition.
(letter from the Greek alphabet)
'teaching alphabet including old' is the wordplay.
'teaching alphabet' becomes 'ITA' (Initial Teaching Alphabet).
'including' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ita' placed around 'o' is 'IOTA'.
(Other definitions for iota that I've seen before include "Jot or tittle" , "Just a jot, from Greek alphabet" , "Ninth Greek character" , "Greek equivalent of the letter I" , "Ninthe letter of the Greek alphabet" .)