Missile I had cast into river (7)
I believe the answer is:
trident
'missile' is the definition.
(UK nuclear weapons system)
'i had cast into river' is the wordplay.
'i had' becomes 'i'd' (contraction).
'cast into' indicates putting letters inside.
'river' becomes 'trent' (River Trent).
'id' placed into 'trent' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "Neptune's weapon" , "Weapon on submarine" , "Submarine-launched ballistic missile" , "Spear held by Neptune or Britannia" , "A three-pronged spear as wielded by Neptune" .)