Angler might use this line by river (4)
I believe the answer is:
lure
'angler might use this' is the definition.
(an angler may use a lure to lure fish)
'line by river' is the wordplay.
'line' becomes 'l'.
'by' means one lot of letters go next to another.
'river' becomes 'ure' (I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure').
'l'+'ure'='LURE'
(Other definitions for lure that I've seen before include "Entice, bait" , "Tempt with some kind of reward" , "'Entice, tempt (4)'" , "Temptation" , "draw" .)