Weapon I'd placed in river (7)
I believe the answer is:
trident
'weapon' is the definition.
(I know that trident is a type of lance)
'i'd placed in river' is the wordplay.
'placed in' is an insertion indicator (I've seen this in other clues).
'river' becomes 'trent' (River Trent).
'id' going into 'trent' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "Pronged spear" , "Fork perhaps" , "Three-pronged weapon" , "3-pronged spear" , "Britannia's weapon" .)