Letter from nine written in reverse (4)
I believe the answer is:
iota
This clue might be a double definition.
'letter' is the first definition.
(Greek letter)
'nine written in reverse' is the second definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'from' is the link.
(Other definitions for iota that I've seen before include "Scintilla" , "A little piece" , "European letter" , "Contrivance" , "Ninth letter of Greek alphabet, small amount" .)