Some light or heavy timber (4)
I believe the answer is:
beam
This might be a double definition.
'some light' is the first definition.
(beam is a kind of light)
'heavy timber' is the second definition.
'beam' can be an answer for 'timber' (I have seen 'Sturdy timber ' mean 'beam' so perhaps 'timber' could also mean 'beam'). I am not certain of the 'heavy' bit.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for beam that I've seen before include "Smile broadly or radiate" , "Perhaps a summer" , "Rafter - smile - width" , "Piece of timber - shaft of light" , "Smile brightly - supporting timber" .)