Beginning to shorten hair causes strain (6)
I believe the answer is:
stress
'beginning' is the definition.
(I know that stress is a type of risk factor)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for stress that I've seen before include "'Worry, pressure (6)'" , "Emphasis on anxiety?" , "possible excuse for sickie?" , "Lay emphasis on - it's a strain" , "possible cause of fracture?" .)