Gunpowder constituent soldiers can put back at front (5)
I believe the answer is:
nitre
'gunpowder constituent' is the definition.
(gunpowder contains nitre)
'soldiers can put back at front' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'can' becomes 'tin' (synonyms).
'put back' says the letters should be written in reverse.
'at front' is a charade indicator (letters next to each other) (some letters go in front of others).
'tin' in reverse letter order is 'nit'.
're' put after 'nit' is 'NITRE'.
(Other definitions for nitre that I've seen before include "constituent of gunpowder" , "explosive component" , "food preservative" , "fertiliser" , "found in a slow match?" .)