Ablett covers defeat in book addendum (8)
I believe the answer is:
glossary
'book addendum' is the definition.
(I've seen this in another clue)
'ablett covers defeat' is the wordplay.
'ablett' becomes 'gary' (I can't justify this - if you can you should believe this answer much more).
'covers' means one lot of letters goes inside another.
'defeat' becomes 'loss' (I've seen this in other clues).
'gary' placed around 'loss' is 'GLOSSARY'.
'in' is the link.
Can you help me to learn more?
(Other definitions for glossary that I've seen before include "Alphabetical list of technical terms" , "A grossly distorted list of special words and definitions" , "Explanation of terms" , "List of words pertaining to a subject" , "Explanation of words" .)