Bishop's headdress (5)
(Other definitions for nitre that I've seen before include "explosive producer?" , "Salt" , "A part of fertilizer" , "fertiliser" , "Otherwise saltpetre" .)
'bishop's headdress' is the definition.
(I've seen this before)
This is all the clue.
(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" .)