State of mental or emotional strain (6)
'state of mental or emotional strain' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stress that I've seen before include "Ictus" , "Particular emphasis" , "Underline - tension" , "Draw attention to" , "Force" .)
(Other definitions for tension that I've seen before include "Stretched, uneasy, condition" , "nerves" , "Stress, tautness" , "Strained state" , "Strained condition" .)