Chap keeps mislaying list of words (8)
I believe the answer is:
glossary
'list of words' is the definition.
(I've seen this in another clue)
'chap keeps mislaying' is the wordplay.
'chap' becomes 'gary' ().
'keeps' is an insertion indicator (inserted letters are kept inside).
'mislaying' becomes 'loss' (loss can mean mislaying something).
'gary' going around 'loss' is 'GLOSSARY'.
(Other definitions for glossary that I've seen before include "List of words with meanings" , "one may see meaning here" , "Explanatory list of terms" , "where terms are laid out" , "List of words pertaining to a subject" .)