Stress: one isn't bothered (7)
I believe the answer is:
tension
'stress' is the definition.
(I know that tension is a type of stress)
'one isn't bothered' is the wordplay.
'bothered' indicates anagramming the letters.
'one'+'isnt'='oneisnt'
'oneisnt' anagrammed gives 'TENSION'.
(Other definitions for tension that I've seen before include "anxious state" , "Strain, nervousness" , "Conflict" , "Strained state" , "Stretched state" .)