Strain is back in part of joint (7)
I believe the answer is:
tension
'strain' is the definition.
(I've seen this before)
'is back in part of joint' is the wordplay.
'back' is a reversal indicator.
'in' means one lot of letters goes inside another.
'part of joint' becomes 'tenon' (I've seen this in other clues).
'is' back-to-front is 'si'.
'si' placed within 'tenon' is 'TENSION'.
(Other definitions for tension that I've seen before include "Mental or emotional strain" , "One isn't (anag.)" , "Stretched, uneasy, condition" , "Stress, tautness" , "Strained condition" .)