Great deal of gold inside bog (6)
I believe the answer is:
morass
'bog' is the definition.
(I know that morass is a type of bog)
'great deal of gold inside' is the wordplay.
'great deal of' becomes 'mass' (I can't justify this - if you can you should believe this answer much more).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'inside' means one lot of letters goes inside another.
'mass' placed around 'or' is 'MORASS'.
Can you help me to learn more?
(Other definitions for morass that I've seen before include "Area of marshy or swampy ground" , "Area of boggy ground" , "Confusion" , "complicated situation" , "Swampy, low-lying land" .)