Light timber (4)
I believe the answer is:
beam
I believe this clue is a double definition.
'light' is the first definition.
(I know that beam is a type of light)
'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 "Smile broadly; plank" , "Wide grin" , "Ray of light - structural piece of wood" , "Smile, transmit (radio signal)" , "Ray of light or a large smile" .)