Spear I had found in river (7)
I believe the answer is:
trident
'spear' is the definition.
(trident is a kind of spear)
'i had found in river' is the wordplay.
'i had' becomes 'id' (I'd).
'found in' indicates putting letters inside.
'river' becomes 'trent' (I've seen this before).
'id' placed inside 'trent' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "Pronged spear" , "Three-pointed spear" , "Spear held by Neptune or Britannia" , "Red tint (anag.)" , "Spear with three prongs" .)