Fool gets engineers explosive ingredient (5)
I believe the answer is:
nitre
'explosive ingredient' is the definition.
(I've seen this before)
'fool gets engineers' is the wordplay.
'fool' becomes 'nit' (both can mean a stupid person).
'gets' says to put letters next to each other.
'engineers' becomes 're' (Royal Engineers).
'nit'+'re'='NITRE'
(Other definitions for nitre that I've seen before include "fertiliser" , "safeguard against decay" , "A part of fertilizer" , "explosive producer?" , "Also known as salt-petre" .)