Greek I (4)
I believe the answer is:
iota
'greek i' 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 "Greek letter; tiny amount" , "Jot, particle" , "Ninth letter of Greek alphabet, small amount" , "The smallest possible amount, or the Greek i" , "Smidgen" .)