Spear fishes beside Welsh river (9)
I believe the answer is:
pikestaff
'spear' is the definition.
(type of spear)
'fishes beside welsh river' is the wordplay.
'fishes' becomes 'pikes' (pike is a kind of fish).
'beside' means one lot of letters go next to another (I've seen this in other clues).
'welsh river' becomes 'taff' (I've seen this before).
'pikes'+'taff'='PIKESTAFF'
(Other definitions for pikestaff that I've seen before include "As plain as this is obvious" , "Shaft, proverbially plain" , "that's very plain!" , "it's plain, one says" , "wooden rod?" .)