Lump of precious metal (6)
I believe the answer is:
nugget
'lump of precious metal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nugget that I've seen before include "Small lump (of something valuable)" , "Lump (of gold, etc)" , "Rough lump of gold" , "Lump of metal, say gold" , "Piece of advice" .)