Understand after weapon turned up -- valuable little piece (6)
I believe the answer is:
nugget
'valuable little piece' is the definition.
(I've seen this before)
'understand after weapon turned up' is the wordplay.
'understand' becomes 'get' (getting is a kind of understanding).
'after' means one lot of letters go next to another.
'weapon' becomes 'gun' (gun is a kind of weapon).
'turned up' says the letters should be written backwards.
'gun' in reverse letter order is 'nug'.
'get' put after 'nug' is 'NUGGET'.
(Other definitions for nugget that I've seen before include "(Small) lump (of value)" , "Lump of, eg, gold" , "Rough lump of gold" , "Piece of gold, information" , "Get gun to the lump" .)