It's cast when light is blocked (6)
I believe the answer is:
shadow
'it's cast when light is blocked' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for shadow that I've seen before include "Dark area from which light is blocked" , "Dark area - referring to parliamentary opposition" , "Trace - dog" , "Inseparable companion; dark patch" , "Dark area, weak remnant (of something)" .)