Swamp could produce nugget containing gold (6)
I believe the answer is:
morass
'swamp' is the definition.
(I've seen this before)
'nugget containing gold' is the wordplay.
'nugget' becomes 'mass' (nugget can mean a mass of something).
'containing' means one lot of letters goes inside another.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'mass' enclosing 'or' is 'MORASS'.
'could produce' acts as a link.
(Other definitions for morass that I've seen before include "Slough - marsh" , "Bog or marsh" , "Tract of soft, wet ground" , "mess" , "Slough, bog" .)