Manure turned over containing nitrogen (4)
I believe the answer is:
dung
'manure' is the definition.
(manure is dung used as fertiliser)
'turned over containing nitrogen' is the wordplay.
'turned over' becomes 'dug' ('dig' can be a synonym of 'turn over').
'containing' means one lot of letters goes inside another.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'dug' placed around 'n' is 'DUNG'.
(Other definitions for dung that I've seen before include "Animal manure" , "Animal excrement" .)