Lay emphasis on the strain (6)
I believe the answer is:
stress
This could be a double definition.
'lay emphasis' is the first definition.
'stress' can be an answer for 'emphasis' (stress is a kind of emphasis). I am unsure of the 'lay' bit.
'the strain' is the second definition.
('stress' can be a synonym of 'strain')
'on' acts as a link.
Can you help me to learn more?
(Other definitions for stress that I've seen before include "Tension - accent" , "Tension; emphasis" , "nervous tension" , "'Worry, pressure (6)'" , "what causes tension" .)