Flower birds turn up on river (8)
I believe the answer is:
larkspur
'flower' is the definition.
(larkspur is a kind of flower)
'birds turn up on river' is the wordplay.
'birds' becomes 'larks' (lark is a kind of bird**).
'turn' says the letters should be written in reverse.
'up on' becomes 'up' ('up' can be similar in meaning to 'up on').
'river' becomes 'r'.
'up' back-to-front is 'pu'.
'larks'+'pu'+'r'='LARKSPUR'
(Other definitions for larkspur that I've seen before include "in bed!" , "Flower like a delphinium" , "Delphinium-like flower" , "it grows" , "Cultivated plant of the genus Delphinium" .)