Three-pronged weapon one day found in river (7)
I believe the answer is:
trident
'three-pronged weapon' is the definition.
(I've seen this before)
'one day found in river' is the wordplay.
'one' becomes 'i' (Roman numeral).
'day' becomes 'd' (abbreviation for day).
'found in' is an insertion indicator.
'river' becomes 'trent' (River Trent).
'i'+'d'='id'
'id' inserted within 'trent' is 'TRIDENT'.
(Other definitions for trident that I've seen before include "Neptune's weapon" , "Red tint (anag.)" , "not pike" , "Neptune's fork" , "Spear with three prongs" .)