Old firearm London police found round river (6)
I believe the answer is:
musket
'old firearm' is the definition.
(I've seen this before)
'london police found round river' is the wordplay.
'london police' becomes 'met' (London Metropolitan Police).
'found round' is an insertion indicator.
'river' becomes 'usk' (Welsh river).
'met' going around 'usk' is 'MUSKET'.
(Other definitions for musket that I've seen before include "[DRAGON]" , "Old infantry weapon" , "A very old rifle" , "Ancient shoulder-gun" , "Muzzle-loading shoulder gun" .)