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 in another clue)
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" .)