A scrap from a letter (4)
I believe the answer is:
iota
I believe this clue is a double definition.
'a scrap' is the first definition.
(both can mean a very small amount)
'a letter' is the second definition.
(Greek letter)
'from' acts as a link.
(Other definitions for iota that I've seen before include "Smidgeon" , "Hint" , "Jot or tittle" , "A little piece" , "Trace" .)