Bit of moonlight (4)
I believe the answer is:
beam
'bit of moonlight' 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 "Smile brightly - supporting timber" , "Apparatus used by women gymnasts" , "Ray - side" , "Piece of timber - shaft of light" , "Ray - large piece of wood" .)