Spear I'd thrown in river (7)
I believe the answer is:
trident
'spear' is the definition.
(I know that trident is a type of spear)
'i'd thrown in river' is the wordplay.
'thrown in' means one lot of letters goes inside another.
'river' becomes 'trent' (River Trent).
'id' inserted into 'trent' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "Neptune's weapon" , "Red tint (anag.)" , "Weapon on submarine" , "Spear held by Neptune or Britannia" , "not pike" .)