Nervous strain (7)
I believe the answer is:
tension
'nervous strain' is the definition.
(I know that tension is a type of nervous strain)
This is all the clue.
(Other definitions for tension that I've seen before include "Intones (anag) - stress" , "Mental or emotional strain" , "Uneasiness" , "Assay" , "The effect of force exerted in or on something" .)