Speak forcefully, providing a cause of tension (6)
I believe the answer is:
stress
'tension' is the definition.
(I know that tension can be written as stress)
I cannot 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 "suffering" , "Emphasize" , "nervous tension" , "Underline - tension" , "Tension, pressure" .)