Fool gives engineers saltpetre (5)
I believe the answer is:
nitre
'saltpetre' is the definition.
(I know that saltpetre can be written as nitre)
'fool gives engineers' is the wordplay.
I cannot quite see how this works, but
'fool' could be 'nit' (both can mean a stupid person) and 'nit' is found in the answer.
'engineers' could be 're' (Royal Engineers) and 're' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nitre that I've seen before include "food preservative" , "Also known as salt-petre" , "Chemical" , "Salt" , "Otherwise saltpetre" .)