Supporter is interrupting scrap (5)
I believe the answer is:
'supporter' is the definition.
(something which supports a building's structure)
'is interrupting scrap' is the wordplay.
'interrupting' is an insertion indicator.
'scrap' becomes 'jot' (I've seen this before).
'is' inserted into 'jot' is 'JOIST'.
(Other definitions for joist that I've seen before include "Timber or steel roof or floor support" , "Beam supporting floors and roofs" .)