Worry one isn't exercising (7)
I believe the answer is:
tension
'worry' is the definition.
(both can mean stress or unease)
'one isn't exercising' is the wordplay.
'exercising' indicates an anagram (letters exercise or move around).
'one'+'isnt'='oneisnt'
'oneisnt' with letters rearranged gives 'TENSION'.
(Other definitions for tension that I've seen before include "State of strain or suspense" , "The effect of force exerted in or on something" , "Conflict" , "Mental or emotional strain" , "Stress, tautness" .)