Smile broadly, seeing supporting member (4)
I believe the answer is:
beam
This clue is a double definition.
'smile broadly' is the first definition.
(I've seen this before)
'supporting member' is the second definition.
'seeing' is the link.
(Other definitions for beam that I've seen before include "XWD SETTER" , "Smile; piece of timber" , "Ray of light or a large smile" , "Big smile; ray" , "Give a wide smile" .)