Pulse started with heart bypass (4)
I believe the answer is:
bean
'pulse' is the definition.
(beans are a type of pulse)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for bean that I've seen before include "See 12 Down" , "comic character" , "Vegetable, could be green or broad" , "coffee, perhaps" , "Plant with seed in pod" .)