Headlight light (4)
I believe the answer is:
beam
'headlight light' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for beam that I've seen before include "Light - heavy timber - wide smile" , "Smile brightly - supporting timber" , "Ray of light or a large smile" , "Ray; piece of timber" , "Smile broadly; plank" .)