Marsh has iron and nitrogen (3)
I believe the answer is:
fen
'marsh' is the definition.
(synonyms)
'iron and nitrogen' is the wordplay.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'and' is a charade indicator (letters next to each other).
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'fe'+'n'='FEN'
'has' is the link.
(Other definitions for fen that I've seen before include "Water-logged area of vegetation" , "Marshland" , "Low marshy land" , "Low marsh" , "Low marshy area" .)