Stratford flower seller, America's rising star (9)
I believe the answer is:
supernova
'star' is the definition.
(supernova is a kind of star)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for supernova that I've seen before include "Star made brilliant by stellar explosion" , "Exploding star that becomes much brighter" , "Star in decline" , "Exploding star that becomes extremely luminous" , "Star that explodes and becomes extremely luminous" .)