Fool concerning explosive ingredient (5)
I believe the answer is:
nitre
'explosive ingredient' is the definition.
(I've seen this before)
'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 "Salt" , "Chemical" , "explosive ingredient" , "food preservative" , "Sodium nitrate" .)