It's easily taken to be illuminating (5)
I believe the answer is:
light
'illuminating' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I can't see?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for light that I've seen before include "What smoker may be requesting" , "Not heavy; illuminate" , "Window; insubstantial" , "Low in density" , "Not dark; not heavy" .)