Rubbish! No one knocks back gold bars (7)
I believe the answer is:
bullion
'gold bars' is the definition.
(I've seen this before)
'rubbish no one knocks back' is the wordplay.
'rubbish' becomes 'bull' (both can mean nonsense).
'one' becomes 'i' (Roman numeral).
'knocks back' says the letters should be written backwards (I've seen 'knock back' mean this).
'no'+'i'='noi'
'noi' reversed gives 'ion'.
'bull'+'ion'='BULLION'
(Other definitions for bullion that I've seen before include "Precious metal in mass" , "expensive bars" , "eg Gold bars" , "Gold or silver in bars or ingots" , "Bars of precious metal" .)