Blackbird left on the river (5)
I believe the answer is:
ousel
'blackbird' is the definition.
(I know that blackbird can be written as ousel)
'left on the river' is the wordplay.
'left' becomes 'l' (common abbreviation).
'on' means one lot of letters go next to another.
'the river' becomes 'ouse' (Yorkshire river).
'l' put after 'ouse' is 'OUSEL'.
(Other definitions for ousel that I've seen before include "Old name for a blackbird; also a kind of thrush" , "Louse turned out to be a thrush (5)." , "Winger" , "Type of thrush" , "Bird; louse (anag.)" .)