Banking system to fix returns -- zero (4)
I believe the answer is:
giro
'banking system' is the definition.
(I've seen this before)
'fix returns zero' is the wordplay.
'fix' becomes 'rig' (rigging is a kind of fixing).
'returns' says the letters should be written in reverse.
'zero' becomes 'o'.
'rig' reversed gives 'gir'.
'gir'+'o'='GIRO'
'to' acts as a link.
(Other definitions for giro that I've seen before include "Cheque" , "Credit transfer; sounds like stable spinning wheel" , "dish from Greece" , "System for monetary transfers used for unemployment benefit etc" , "Banking system" .)