I had one set of books: one chapter is silly (7)
I believe the answer is:
idiotic
'silly' is the definition.
(I've seen this before)
'i had one set of books one chapter' is the wordplay.
'i had' becomes 'i'd' (contraction).
'one' becomes 'i' (Roman numeral).
'set of books' becomes 'OT' (abbreviation for the Old Testament).
'one' becomes 'i' (Roman numeral).
'chapter' becomes 'c' (abbreviation).
'id'+'i'+'ot'+'i'+'c'='IDIOTIC'
'is' acts as a link.
(Other definitions for idiotic that I've seen before include "Silly" , "Witless" , "Inviting ridicule" , "description of Dostoevsky hero?" , "without any point" .)