Pulse started, heart failing (4)
I believe the answer is:
bean
'pulse' is the definition.
(beans are a type of pulse)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for bean that I've seen before include "Seed of coffee plant" , "Seed of e.g. coffee" , "kept by the counter?" , "Edible seed" , "It's often baked" .)