Boring book about paintings (8)
I believe the answer is:
toilsome
'boring' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?
'book about paintings' is the wordplay.
'book' becomes 'tome' (a tome is a book).
'about' means one lot of letters goes inside another.
'paintings' becomes 'oils' (oil paintings).
'tome' going around 'oils' is 'TOILSOME'.
Can you help me to learn more?
(Other definitions for toilsome that I've seen before include "needing much work" , "Needing much effort" , "laborious" .)