Missile's explosive contains free energy (7)
I believe the answer is:
trident
'missile's' is the definition.
(I've seen this before)
'explosive contains free energy' is the wordplay.
'explosive' becomes 'tnt' (TNT is an example).
'contains' means one lot of letters goes inside another.
'free energy' becomes 'ride' (I can't explain this - if you can you should believe this answer much more).
'tnt' placed around 'ride' is 'TRIDENT'.
Can you help me to learn more?
(Other definitions for trident that I've seen before include "Fork perhaps" , "Spear with three prongs" , "Spear held by Neptune or Britannia" , "Red tint (anag.)" , "Neptune's spear - missile" .)