Rubbish book found in outbuilding (7)
I believe the answer is:
garbage
'rubbish' is the definition.
(both can mean refuse or trash)
'book found in outbuilding' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'found in' means one lot of letters goes inside another.
'outbuilding' becomes 'garage' (garage is a kind of outbuilding).
'b' put inside 'garage' is 'GARBAGE'.
(Other definitions for garbage that I've seen before include "Refuse - worthless talk" , "Waste, refuse" , "Food waste or trash" , "US rubbish" , "Brag about age? - that's rubbish" .)