Bishop's headdress (5)
(Other definitions for nitre that I've seen before include "explosive producer?" , "Salt" , "A part of fertilizer" , "fertiliser" , "Otherwise saltpetre" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">bishop's headdress</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 mitre that I've seen before include "One identifies bishop" , "Type of joint - bishop's hat" , "A bishop's tall hat" , "Bishop's cap - wood joint" , "Bishop's headwear" .)