Supporting member making us smile broadly (4)
I believe the answer is:
beam
This clue is a double definition.
'supporting member' is the first definition.
'smile broadly' is the second definition.
(I've seen this before)
'making us' acts as a link.
(Other definitions for beam that I've seen before include "Ray - side" , "A broad smile like lunar ray" , "Ray; piece of timber" , "Rafter - smile - width" , "Ray of light or a large smile" .)