Explosive material in container brought back by sappers (5)
I believe the answer is:
nitre
'explosive material' is the definition.
(term for the explosive saltpetre)
'container brought back by sappers' is the wordplay.
'container' becomes 'tin' (tin is a kind of container).
'brought back' says the letters should be written backwards.
'by' is a charade indicator (letters next to each other).
'sappers' becomes 're' (Royal Engineers).
'tin' back-to-front is 'nit'.
'nit'+'re'='NITRE'
'in' is the link.
(Other definitions for nitre that I've seen before include "explosive component" , "found in a slow match?" , "explosive producer?" , "Otherwise saltpetre" , "A part of fertilizer" .)