Missile shelter offering protection for free (7)
I believe the answer is:
trident
'missile' is the definition.
(UK nuclear weapons system)
'shelter offering protection for free' is the wordplay.
'shelter offering' becomes 'tent' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'protection for' is an insertion indicator (some letters go around or protect others).
'free' becomes 'rid' (to be free of something is to be rid of it).
'tent' enclosing 'rid' is 'TRIDENT'.
Can you help me to learn more?
(Other definitions for trident that I've seen before include "spear carried at sea" , "Spear held by Neptune or Britannia" , "Weapon on submarine" , "Pronged spear" , "Spear with three prongs" .)