Land's ground seemed to contain nitrogen (7)
I believe the answer is:
demesne
'land's' is the definition.
(synonyms)
'ground seemed to contain nitrogen' is the wordplay.
'ground' indicates anagramming the letters ('grind' the letters into a new form).
'to contain' means one lot of letters goes inside another.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'seemed' anagrammed gives 'demese'.
'demese' placed around 'n' is 'DEMESNE'.
(Other definitions for demesne that I've seen before include "Domain" , "Large landed property" , "Seed men on landed estate" , "Extensive landed property" , "Land attached to manor" .)