Fool regarding explosive ingredient (5)
I believe the answer is:
nitre
'explosive ingredient' is the definition.
(I've seen this before)
'fool regarding' is the wordplay.
'fool' becomes 'nit' (both can mean a stupid person).
'regarding' becomes 're' ('re' means regarding or about).
'nit'+'re'='NITRE'
(Other definitions for nitre that I've seen before include "A part of fertilizer" , "Gunpowder ingredient" , "explosive ingredient" , "found in a slow match?" , "Inert kind of saltpetre" .)