Spear I’d used in river (7)
I believe the answer is:
trident
'spear' is the definition.
(trident is a kind of spear)
'i'd used in river' is the wordplay.
'used in' indicates putting letters inside.
'river' becomes 'trent' (I've seen this before).
'id' going inside 'trent' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "Submarine-launched ballistic missile" , "A three-pronged spear as wielded by Neptune" , "Weapon on submarine" , "Neptune's weapon" , "Nuclear missile" .)