Fool concerning explosive compound (5)
I believe the answer is:
nitre
'explosive compound' is the definition.
'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 "Inter (anag.)" , "explosive ingredient" , "what goes into bangers" , "explosive producer?" , "Mineral used in gunpowder" .)