Fool engineers getting an explosive ingredient (5)
I believe the answer is:
nitre
'an explosive ingredient' is the definition.
(I've seen this before)
'fool engineers getting' is the wordplay.
'fool' becomes 'nit' (both can mean a stupid person).
'engineers' becomes 're' (Royal Engineers).
'getting' says to put letters next to each other.
'nit'+'re'='NITRE'
(Other definitions for nitre that I've seen before include "constituent of gunpowder" , "Salt" , "Chemical" , "Gunpowder ingredient" , "explosive component" .)