State branch has energy weapon (5)
I believe the answer is:
sabre
'weapon' is the definition.
(sabre is a kind of weapon)
'state branch has energy' is the wordplay.
'state' becomes 'sa' (South Australia).
'branch' becomes 'br'.
'has' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mca).
'sa'+'br'+'e'='SABRE'
(Other definitions for sabre that I've seen before include "Walt --, cartoonist" , "weapon" , "Large wasp" , "Heavy cavalry sword" , "Bares (anag.)" .)