Emphasis on strain (6)
I believe the answer is:
stress
This clue is a double definition.
'emphasis' is the first definition.
(I know that stress is a type of emphasis)
'strain' is the second definition.
(I know that stress is a type of strain)
'on' acts as a link.
(Other definitions for stress that I've seen before include "Emphasise the strain" , "Emphasis: result often of being under pressure" , "Tension; emphasis" , "Emphasize" , "Emotional tension" .)