Gold found in lump in swamp (6)
I believe the answer is:
morass
'swamp' is the definition.
(thesaurus)
'gold found in lump' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'found in' means one lot of letters goes inside another.
'lump' becomes 'mass' (I have seen 'Lump of material' mean 'mass' so perhaps 'lump' could also mean 'mass').
'or' inserted into 'mass' is 'MORASS'.
'in' acts as a link.
(Other definitions for morass that I've seen before include "Mire, quagmire" , "Muddled situation" , "Bog or swamp" , "mess" , "Swampy, low-lying land" .)