Of little physical weight (5)
I believe the answer is:
light
(Other definitions for light that I've seen before include "Window; insubstantial" , "Mild" , "Ignite; of little weight" , "Source of illumination" , "answer to clue" .)
I believe the answer is:
light
(Other definitions for light that I've seen before include "Window; insubstantial" , "Mild" , "Ignite; of little weight" , "Source of illumination" , "answer to clue" .)