Gunpowder ingredient (5)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gunpowder ingredient</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for nitre that I've seen before include "explosive component" , "Inert (anag) - saltpetre" , "Chemical" , "food preservative" , "Sodium nitrate" .)