Rubbish religious books in Barnet (3,3)
I believe the answer is:
hot air
'rubbish' is the definition.
('hot air' can mean rubbish or nonsense)
'religious books in barnet' is the wordplay.
'religious books' becomes 'OT' (Old Testament).
'in' indicates putting letters inside.
'barnet' becomes 'hair' (Barnet Fair is Cockney rhyming slang for hair).
'ot' inserted within 'hair' is 'HOT-AIR'.
(Other definitions for hot air that I've seen before include "Talking nonsense" , "Empty boastful talk; balloon lift provider" , "Guff" , "Bombastic talk or balloon elevator" , "nothing worth listening to" .)