Stress numbers to charge (7)
I believe the answer is:
tension
'stress' is the definition.
(I know that tension is a type of stress)
'numbers to charge' is the wordplay.
'numbers' becomes 'tens' (ten is a kind of number).
'to charge' becomes 'ion' (an ion is a charged particle).
'tens'+'ion'='TENSION'
(Other definitions for tension that I've seen before include "Conflict" , "A pulling strain" , "Nervous strain" , "Strain, nervousness" , "Strained state" .)