I'd plunged into river to find spear (7)
I believe the answer is:
trident
'spear' is the definition.
(trident is a kind of spear)
'i'd plunged into river' is the wordplay.
'plunged into' indicates putting letters inside.
'river' becomes 'trent' (I've seen this before).
'id' put within 'trent' is 'TRIDENT'.
'to find' is the link.
(Other definitions for trident that I've seen before include "Britannia's weapon" , "Pronged spear" , "Nuclear missile" , "Neptune's spear - missile" , "Weapon on submarine" .)