Numbers working under current stress (7)
I believe the answer is:
tension
'stress' is the definition.
('tension' can be a synonym of 'stress')
'numbers working under current' is the wordplay.
'numbers' becomes 'tens' (I have seen 'Cardinal numbers ' mean 'tens' so perhaps 'numbers' could also mean 'tens'**).
'working' becomes 'on'.
'under' means one lot of letters go next to another.
'current' becomes 'i' (symbol for electric current).
'on' after 'i' is 'ion'.
'tens'+'ion'='TENSION'
(Other definitions for tension that I've seen before include "State of strain or suspense" , "Stretched, uneasy, condition" , "Uneasiness" , "The effect of force exerted in or on something" , "A pulling strain" .)