Bit of a letter, final bit in rambling screed (9)

'bit' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?

'letter final bit in rambling screed' is the wordplay.
'letter' becomes 'd'.
'final bit' becomes 'en' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'rambling' indicates an anagram.
'screed' with letters rearranged gives 'escder'.
'en' placed into 'escder' is 'escender'.
'd'+'escender'='DESCENDER'

'of a' acts as a link.

Can you help me to learn more?