The force of emphasis (6)
I believe the answer is:
stress
I believe this is a double definition.
'the force' is the first definition.
(stress is a kind of force)
'emphasis' is the second definition.
(I know that stress is a type of emphasis)
'of' is the link.
(Other definitions for stress that I've seen before include "Force" , "Emphasis on anxiety?" , "Emphasis: result often of being under pressure" , "Pressure or strain" , "result of being overworked" .)