Weapon made of explosive containing free energy (7)
I believe the answer is:
trident
'weapon' is the definition.
(trident is a kind of weapon)
'of explosive containing free energy' is the wordplay.
'of explosive' becomes 'tnt' (I can't justify this - if you can you should give a lot more credence to this answer).
'containing' means one lot of letters goes inside another.
'free' becomes 'rid' ('rid' can be a synonym of 'free').
'energy' becomes 'e' (physics symbol as in E=mcn).
'rid'+'e'='ride'
'tnt' enclosing 'ride' is 'TRIDENT'.
'made' is the link.
Can you help me to learn more?
(Other definitions for trident that I've seen before include "Nuclear weapon" , "Three-pronged weapon" , "Red tint? (anagram)" , "Red tint (anag.)" , "Neptune's or Britannia's spear" .)