Smile; timber (4)
I believe the answer is:
beam
I believe this clue is a double definition.
'smile' is the first definition.
(to beam is to smile)
'timber' is the second definition.
(I have seen 'Sturdy timber ' mean 'beam' so perhaps 'timber' could also mean 'beam')
(Other definitions for beam that I've seen before include "Ray of light or a large smile" , "Sturdy timber" , "A supportive" , "Smile, light shaft" , "Rafter - smile - width" .)