Gold discovered in extensive marsh (6)
I believe the answer is:
morass
'marsh' is the definition.
(thesaurus)
'gold discovered in extensive' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'discovered in' means one lot of letters goes inside another.
'extensive' becomes 'mass' (as in mass production or mass media).
'or' inserted into 'mass' is 'MORASS'.
(Other definitions for morass that I've seen before include "Swampy, low-lying land" , "Bog or marsh" , "Bog; confused situation" , "Bog - situation hard to get out of" , "Tract of soft, wet ground" .)