Idiot gets engineers an explosive compound (5)
I believe the answer is:
nitre
'an explosive compound' is the definition.
'idiot gets engineers' is the wordplay.
'idiot' becomes 'nit' (nit can mean a stupid person).
'gets' says to put letters next to each other.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'nit'+'re'='NITRE'
(Other definitions for nitre that I've seen before include "A part of fertilizer" , "what goes into bangers" , "explosive ingredient" , "safeguard against decay" , "Inert kind of saltpetre" .)