Lump of gold (6)
I believe the answer is:
nugget
'lump of gold' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nugget that I've seen before include "Lump of, eg, gold" , "Lump of valuable metal" , "Lump sought by the prospector" , "small, valuable item" , "Small lump (of something valuable)" .)