One isn't affected by stress (7)
I believe the answer is:
tension
'stress' is the definition.
(synonyms)
'one isn't affected' is the wordplay.
'affected' indicates anagramming the letters.
'one'+'isnt'='oneisnt'
'oneisnt' with letters rearranged gives 'TENSION'.
'by' is the 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" .)