Strain or tension (6)
I believe the answer is:
stress
This clue is a double definition.
'strain' is the first definition.
(I know that strain can be written as stress)
'tension' is the second definition.
(I know that tension can be written as stress)
(Other definitions for stress that I've seen before include "Force" , "State mental or emotional strain" , "Put weight on" , "Emphasize" , "Pressure or strain" .)