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 in another clue)
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" .)