It's neither dark nor heavy (5)
I believe the answer is:
light
'heavy' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for light that I've seen before include "Not dark; not heavy" , "Mild" , "Source of illumination" , "without difficulty" , "Unimportant" .)