Spear papers dropped in river (7)
I believe the answer is:
trident
'spear' is the definition.
(trident is a kind of spear)
'papers dropped in river' is the wordplay.
'papers' becomes 'id' (identification papers).
'dropped in' indicates putting letters inside.
'river' becomes 'trent' (I've seen this before).
'id' going into 'trent' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "A three-pronged spear as wielded by Neptune" , "3-pronged spear" , "spear carried at sea" , "Pronged spear" , "Neptune's or Britannia's spear" .)