Business undertaking not easily disturbed (4)
I believe the answer is:
firm
'business' is the definition.
(I know that firm is a type of business)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for firm that I've seen before include "Steady and unyielding, like business organisation" , "securely rooted" , "team of doctors and assistants" , "team of doctors in hospital" , "Company; steady" .)