Ward Duke off in marshland (3)
I believe the answer is:
fen
'marshland' is the definition.
(fen is a type of wetland)
'ward duke off' is the wordplay.
'ward' becomes 'fend' (as in warding or fending off an attacker).
'duke' becomes 'd' (abbreviation).
'off' suggests deleting specific letters.
'fend' with 'd' removed is 'FEN'.
'in' is the link.
(Other definitions for fen that I've seen before include "Slough" , "Low-lying wet land with grassy vegetation" , "Marshy area of land" , "Low marshy area" , "Low marsh" .)