Louse regarding saltpetre (5)
I believe the answer is:
nitre
'saltpetre' is the definition.
(I know that saltpetre can be written as nitre)
'louse regarding' is the wordplay.
'louse' becomes 'nit' (I've seen this in another clue).
'regarding' becomes 're' (re can mean regarding, concerning).
'nit'+'re'='NITRE'
(Other definitions for nitre that I've seen before include "Inert (anag) - saltpetre" , "safeguard against decay" , "Also known as salt-petre" , "Inert kind of saltpetre" , "food preservative" .)