Firearm found in river collected by police (6)
I believe the answer is:
musket
'firearm' is the definition.
(musket is a kind of firearm)
'river collected by police' is the wordplay.
'river' becomes 'usk' (Welsh river).
'collected by' indicates putting letters inside.
'police' becomes 'met' (I've seen this in other clues).
'usk' placed within 'met' is 'MUSKET'.
'found in' acts as a link.
(Other definitions for musket that I've seen before include "Shoulder gun with a long barrel" , "A very old rifle" , "Long-barrelled gun" , "Old infantry weapon" , "Shooter" .)