Strangely, one isn't showing stress (7)
I believe the answer is:
tension
'stress' is the definition.
('tension' can be a synonym of 'stress')
'strangely one isn't' is the wordplay.
'strangely' indicates anagramming the letters.
'one'+'isnt'='oneisnt'
'oneisnt' is an anagram of 'TENSION'.
'showing' is the link.
(Other definitions for tension that I've seen before include "Conflict" , "anxious state" , "Strained state" , "Stretched, uneasy, condition" , "Strain, nervousness" .)