Not much of a title, maybe, for the letter heading (6)
I believe the answer is:
little
'not much' is the definition.
('little' is the opposite to 'much')
'a title maybe for the letter heading' is the wordplay.
'maybe' is an anagram indicator.
'for' means one lot of letters go next to another (I've seen this in other clues).
'the letter heading' becomes 'l' (I can't justify this - if you can you should believe this answer much more).
'title' with letters rearranged gives 'ittle'.
'ittle' after 'l' is 'LITTLE'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for little that I've seen before include "Piddling" , "(Heroine of) Dickens novel" , "Jobsworth?" , "Peanuts" , "Tell it (anag) - not a lot!" .)