Notice earth and mire around rider? (8)
I believe the answer is:
addendum
'rider?' is the definition.
(thesaurus)
'notice earth and mire around' is the wordplay.
'notice earth' becomes 'nedda' (I can't justify this - if you can you should give a lot more credence to this answer).
'and' is a charade indicator (letters next to each other).
'mire' becomes 'mud' (synonyms).
'around' says the letters should be written backwards.
'nedda' after 'mud' is 'mudnedda'.
'mudnedda' back-to-front is 'ADDENDUM'.
Can you help me to learn more?
(Other definitions for addendum that I've seen before include "Textual matter added on" , "Something attached to the end, say of book" , "Material added to the end of a book" , "Something appended as to end of book" , "Extra item added at end of book" .)