One isn’t affected by stress (7)
I believe the answer is:
'stress' is the definition.
'one isn't affected' is the wordplay.
'affected' is an anagram indicator.
'oneisnt' anagrammed gives 'TENSION'.
'by' acts as a link.
(Other definitions for tension that I've seen before include "The effect of force exerted in or on something" , "anxious state" , "nerves" , "Strained condition" , "Intones (anag) - stress" .)