Gold found in extensive swamp (6)
I believe the answer is:
morass
'swamp' is the definition.
(thesaurus)
'gold found in extensive' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'found in' is an insertion indicator.
'extensive' becomes 'mass' (as in mass production or mass media).
'or' put inside 'mass' is 'MORASS'.
(Other definitions for morass that I've seen before include "Swampy, low-lying land" , "Bog; confused situation" , "dangerous situation" , "Bog or swamp" , "Mire, quagmire" .)