Having insufficient lighting is stupid (3)
I believe the answer is:
dim
'is stupid' is the definition.
(similar in meaning)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for dim that I've seen before include "Gloomy; dull" , "Without much light" , "reduce volume" , "Not clear or not too bright" , "Vague" .)