Features records written up in sacred book (7)
I believe the answer is:
aspects
'features' is the definition.
(aspect is a kind of feature)
'records written up in sacred book' is the wordplay.
'records' becomes 'eps' (EP is a type of record).
'written up' says the letters should be written backwards.
'in' means one lot of letters goes inside another.
'sacred book' becomes 'acts' (book of the New Testament).
'eps' backwards is 'spe'.
'spe' going inside 'acts' is 'ASPECTS'.
(Other definitions for aspects that I've seen before include "'Appearances, facets (7)'" , "Looks or appearances" , "Appearance, facet" , "Looks, scenes, vistas" , "Points of view" .)