Having brought up weapon, secure a lump of gold (6)
I believe the answer is:
nugget
'a lump of gold' is the definition.
(I've seen this before)
'having brought up weapon secure' is the wordplay.
'having brought up' says the letters should be written in reverse (in a down clue, letters go up).
'weapon' becomes 'gun' (I've seen this before).
'secure' becomes 'get'.
'gun' in reverse letter order is 'nug'.
'nug'+'get'='NUGGET'
(Other definitions for nugget that I've seen before include "Small lump of gold" , "bit of gold" , "Rough lump of gold" , "Lump of metal, say gold" , "Valuable little piece" .)