Emphasize the strain (6)
I believe the answer is:
stress
This clue is a double definition.
'emphasize' is the first definition.
(I know that emphasize can be written as stress)
'the strain' is the second definition.
(stress is a kind of strain)
(Other definitions for stress that I've seen before include "suffering" , "Emphasize" , "Draw attention to" , "Emphasise the pressure" , "State mental or emotional strain" .)