Returns gun and obtains valuable lumps of metal (7)
I believe the answer is:
nuggets
'lumps of metal' is the definition.
'nuggets' can be an answer for 'lumps' (nugget is a kind of lump). I'm not sure about the 'of metal' bit.
'returns gun and obtains valuable' is the wordplay.
'returns' says the letters should be written in reverse.
'and' says to put letters next to each other.
'obtains valuable' becomes 'gets' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'gun' in reverse letter order is 'nug'.
'nug'+'gets'='NUGGETS'
Can you help me to learn more?
(Other definitions for nuggets that I've seen before include "Lumps of metal like gold" , "Solid lumps of a precious metal" , "Small lumps of gold" .)