Crossing Welsh river, came across old firearm (6)
I believe the answer is:
musket
'old firearm' is the definition.
(I've seen this in another clue)
'crossing welsh river came across' is the wordplay.
'crossing' is an insertion indicator (to cross can mean to go over or around).
'welsh river' becomes 'usk' (River Usk in Wales).
'came across' becomes 'met' (to come across is to meet or encounter).
'usk' put within 'met' is 'MUSKET'.
(Other definitions for musket that I've seen before include "Old type of gun" , "Long-barrelled gun" , "Early infantry firearm" , "Shoulder gun with a long barrel" , "A very old rifle" .)