Ninth letter of the Greek alphabet (4)
I believe the answer is:
iota
'ninth letter of the greek alphabet' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for iota that I've seen before include "little part" , "letter from abroad" , "Just a jot, like Greek letter" , "Greek equivalent of the letter I" , "Hint" .)