Not much of a letter (4)
I believe the answer is:
iota
'letter' is the definition.
(Greek letter)
'not much of a' is the wordplay.
I cannot really understand how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
'a' is found in the answer.
The remaining letters 'it' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for iota that I've seen before include "Greek letter; tiny amount" , "Ninthe letter of the Greek alphabet" , "Tiny or scarcely detectable amount" , "Jot or tittle" , "(Not one) little bit" .)