Strain originally striking one in part of joint (7)
I believe the answer is:
tension
'strain' is the definition.
(I know that tension is a type of strain)
'originally striking one in part of joint' is the wordplay.
'originally' suggests taking the first letters.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'part of joint' becomes 'tenon' (type of wooden joint).
The first letter of 'striking' is 's'.
's'+'i'='si'
'si' put within 'tenon' is 'TENSION'.
(Other definitions for tension that I've seen before include "Strain, nervousness" , "Anxiety" , "Nervous strain" , "State of strain or suspense" , "Conflict" .)