One isn't disturbed by nervous strain (7)
I believe the answer is:
tension
'nervous strain' is the definition.
(I know that tension is a type of nervous strain)
'one isn't disturbed' is the wordplay.
'disturbed' indicates anagramming the letters (the order of letters is disturbed).
'one'+'isnt'='oneisnt'
'oneisnt' with letters rearranged gives 'TENSION'.
'by' is the link.
(Other definitions for tension that I've seen before include "Strained condition" , "Stretched state" , "Nervous strain" , "Uneasiness" , "Conflict" .)