Clue that's illuminating (5)
I believe the answer is:
light
This clue might be a double definition.
'clue' is the first definition.
'illuminating' is the second definition.
Both the answer and definition are adjectives. Maybe there's a link between them I don't understand?
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for light that I've seen before include "solution to clue" , "answer to clue" , "Window; insubstantial" , "Undemanding; crossword answer" , "Ignite; of little weight" .)