Bait left on river (4)
I believe the answer is:
lure
'bait' is the definition.
('lure' can be a synonym of 'bait')
'left on river' is the wordplay.
'left' becomes 'l' (common abbreviation).
'on' 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 "Temptation" , "Attract with bait" , "Type of bait" , "Tempt invoking rule" , "draw" .)