Stress: one isn't relaxed (7)
I believe the answer is:
'stress' is the definition.
'one isn't relaxed' is the wordplay.
'relaxed' indicates an anagram.
'oneisnt' anagrammed gives 'TENSION'.
(Other definitions for tension that I've seen before include "Strain, nervousness" , "Anxiety" , "nerves" , "Intones (anag) - stress" , "anxious state" .)