Turn down, as lights (3)
I believe the answer is:
dim
'turn down as lights' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dim that I've seen before include "Not clear or not too bright" , "Stupid - poorly lit" , "reduce volume" , "Lacking clarity or distinctness" , "Slow-witted" .)