Energy in new brass weapons (6)
I believe the answer is:
sabres
'weapons' is the definition.
(sabre is a kind of weapon)
'energy in new brass' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'in' means one lot of letters goes inside another.
'new' is an anagram indicator.
'brass' anagrammed gives 'sabrs'.
'e' going into 'sabrs' is 'SABRES'.
(Other definitions for sabres that I've seen before include "Stout swords with a curved blades" , "Cavalry swords with curved blades" .)