Strain, emphasis (6)
I believe the answer is:
stress
I believe this clue is a double definition.
'strain' is the first definition.
(I know that stress is a type of strain)
'emphasis' is the second definition.
(I know that stress is a type of emphasis)
(Other definitions for stress that I've seen before include "Tension - accent" , "Underline - tension" , "Lay emphasis on - it's a strain" , "Highlight" , "Force" .)