Tiny bit in letter from Greece (4)
I believe the answer is:
iota
I believe this clue is a double definition.
'tiny bit' is the first definition.
(an iota is a tiny piece)
'letter from greece' is the second definition.
(letter from the Greek alphabet)
'in' is the link.
(Other definitions for iota that I've seen before include "letter from Greece" , "Grain" , "Smidgen" , "Tiny or scarcely detectable amount" , "Ninthe letter of the Greek alphabet" .)