Fool concerning saltpetre (5)
I believe the answer is:
nitre
'saltpetre' is the definition.
(I know that saltpetre can be written as nitre)
'fool concerning' is the wordplay.
'fool' becomes 'nit' (both can mean a stupid person).
'concerning' becomes 're' (both can mean on a particular topic).
'nit'+'re'='NITRE'
(Other definitions for nitre that I've seen before include "what goes into bangers" , "constituent of gunpowder" , "explosive ingredient" , "explosive producer?" , "found in a slow match?" .)