Light line (3)
I believe the answer is:
ray
'light line' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ray that I've seen before include "Flash < Movie Player" , "Pencil of light" , "Broad flat fish; beam" , "Beam of light or flat fish" , "Beam; flat-bodied fish" .)