Suspense is rising in joint (7)
I believe the answer is:
tension
'suspense' is the definition.
(thesaurus)
'is rising in joint' is the wordplay.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'in' means one lot of letters goes inside another.
'joint' becomes 'tenon' (I've seen this before).
'is' in reverse letter order is 'si'.
'si' placed inside 'tenon' is 'TENSION'.
(Other definitions for tension that I've seen before include "nerves" , "State of strain or suspense" , "Strained state" , "Strained condition" , "One isn't (anag.)" .)