How haunted houses are lit (5)
I believe the answer is:
dimly
'how haunted houses are lit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for dimly that I've seen before include "'Faintly, without much light (5)'" , "Indistinctly" , "with little distinction" , "In a vague way" .)