Fool turning up in splendour? This should explain things (8)
I believe the answer is:
glossary
'this should explain things' is the definition.
The definition suggests a singular noun which matches the answer.
'fool turning up in splendour?' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'turning up' says the letters should be written in reverse.
'in' indicates putting letters inside.
'splendour?' becomes 'glory' (glory can mean splendour or grandeur).
'ass' backwards is 'ssa'.
'ssa' going within 'glory' is 'GLOSSARY'.
Can you help me to learn more?
(Other definitions for glossary that I've seen before include "where terms are laid out" , "List of words pertaining to a subject" , "Index of terms" , "List of technical terms in an appendix" , "Alphabetical list of technical terms" .)