Songbird seen on branch line flower (8)
I believe the answer is:
larkspur
'flower' is the definition.
(larkspur is a kind of flower)
'songbird seen on branch line' is the wordplay.
'songbird' becomes 'lark' (I've seen this before).
'seen on' says to put letters next to each other.
'branch line' becomes 'spur' ('spur' can be a synonym of 'branch line').
'lark'+'spur'='LARKSPUR'
(Other definitions for larkspur that I've seen before include "Cultivated plant of the genus Delphinium" , "Mediterranean plant of the buttercup family" , "in bed!" , "Garden flower of delphinium family" , "Flower (named after excrescence on a bird?)" .)