Can return concerned with sodium nitrate (5)
I believe the answer is:
nitre
'sodium nitrate' is the definition.
(I've seen this before)
'can return concerned with' is the wordplay.
'can' becomes 'tin' (both are metal containers).
'return' is a reversal indicator.
'concerned with' becomes 're' (re can mean regarding, concerning).
'tin' written backwards gives 'nit'.
'nit'+'re'='NITRE'
(Other definitions for nitre that I've seen before include "Sodium nitrate" , "safeguard against decay" , "Also known as salt-petre" , "explosive ingredient" , "Gunpowder ingredient" .)