One isn't poorly -- it's stress (7)
I believe the answer is:
tension
'stress' is the definition.
('tension' can be a synonym of 'stress')
'one isn't poorly' is the wordplay.
'poorly' indicates anagramming the letters.
'one'+'isnt'='oneisnt'
'oneisnt' anagrammed gives 'TENSION'.
'it's' acts as a link.
(Other definitions for tension that I've seen before include "anxious state" , "One isn't (anag.)" , "Mental or emotional strain" , "Conflict" , "The effect of force exerted in or on something" .)