I as in Ithaca (4)
I believe the answer is:
iota
'i as in ithaca' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for iota that I've seen before include "Smidgen" , "Just a jot, from Greek alphabet" , "Old letter" , "Scintilla" , "Trace" .)