Birds turned up on river to see plant (8)
I believe the answer is:
larkspur
'plant' is the definition.
(larkspur is a kind of plant)
'birds turned up on river' is the wordplay.
'birds' becomes 'larks' (lark is a kind of bird).
'turned' says the letters should be written in reverse.
'on' says to put letters next to each other.
'river' becomes 'r' (abbreviation for river).
'up' in reverse letter order is 'pu'.
'larks'+'pu'+'r'='LARKSPUR'
'to see' acts as a link.
(Other definitions for larkspur that I've seen before include "Cultivated plant of the genus Delphinium" , "Delphinium-like plant" , "Mediterranean plant with spikes of blue, pink or white flowers" , "Mediterranean plant of the buttercup family" , "Plant with flower spikes" .)