A flower changes, in short, Penelope’s heart (5)
I believe the answer is:
peony
'a flower' is the definition.
(peony is a kind of flower)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for peony that I've seen before include "Garden flower" , "Bloom" , "Herbaceous flower" , "Plant with large red, white or pink flowers" .)