Three-pronged spear I'd put in river (7)
I believe the answer is:
trident
'three-pronged spear' is the definition.
(I've seen this before)
'i'd put in river' is the wordplay.
'put in' is an insertion indicator.
'river' becomes 'trent' (River Trent).
'id' put inside 'trent' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "Submarine-launched ballistic missile" , "Nuclear missile" , "Britannia's weapon" , "Spear held by Neptune or Britannia" , "Red tint? (anagram)" .)