Nature god banking silver for heathen (5)
I believe the answer is:
pagan
'heathen' is the definition.
(both can mean an irreligious person)
'nature god banking silver' is the wordplay.
'nature god' becomes 'Pan' (Pan is a god in Greek mythology).
'banking' indicates putting letters inside.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'pan' going around 'ag' is 'PAGAN'.
'for' is the link.
(Other definitions for pagan that I've seen before include "advocate of pleasures" , "Unchristian" , "Unbeliever" , "Non-mainstream religious believer" , "One who does not acknowledge a god" .)