Fool engineers explosive compound (5)
I believe the answer is:
nitre
'explosive compound' is the definition.
'fool engineers' is the wordplay.
'fool' becomes 'nit' (both can mean a stupid person).
'engineers' becomes 're' (Royal Engineers).
'nit'+'re'='NITRE'
(Other definitions for nitre that I've seen before include "Inter (anag.)" , "what goes into bangers" , "explosive ingredient" , "Also known as salt-petre" , "Chemical" .)