Form of nitre put into the ground (5)
I believe the answer is:
inter
'put into the ground' is the definition.
(I've seen this in another clue)
'form of nitre' is the wordplay.
'form of' is an anagram indicator (the letters need to be in another 'form').
'nitre' anagrammed gives 'INTER'.
(Other definitions for inter that I've seen before include "Lay to rest" , "Bury - Milan football club" , "Inhume" , "Nitre (anag.)" , "Prefix meaning between or among" .)