Diner supplied sandwiches, building support (8)
I believe the answer is:
scaffold
'building support' is the definition.
'diner supplied sandwiches' is the wordplay.
'diner' becomes 'caff' (a cafe or 'caff' is a type of diner).
'supplied' becomes 'sold'.
'sandwiches' is an insertion indicator (to sandwich is to block something in).
'caff' placed within 'sold' is 'SCAFFOLD'.
(Other definitions for scaffold that I've seen before include "Execution platform" , "support" , "Place of execution" , "Platform on which criminals were executed" , "Temporary erection for workmen - gallows" .)