Strangely one isn't under stress (7)
I believe the answer is:
tension
'stress' is the definition.
(synonyms)
'strangely one isn't under' is the wordplay.
'strangely' indicates anagramming the letters.
'under' is a charade indicator (letters next to each other).
'one'+'isnt'='oneisnt'
'oneisnt' with letters rearranged gives 'TENSION'.
(Other definitions for tension that I've seen before include "nerves" , "Strain, nervousness" , "State of strain or suspense" , "A pulling strain" , "Stretched state" .)