A leaf’s outline reproduced by a plant (7)
'plant' is the definition.
(alfalfa is a kind of plant)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for alfalfa that I've seen before include "Lucerne, a fodder plant", "odder plant", "Plant used as fodder", "A forage plant", "food plant".)