Insult lights by mistake (6)
I believe the answer is:
slight
'insult' is the definition.
(thesaurus)
'lights by mistake' is the wordplay.
'by mistake' is an anagram indicator.
'lights' anagrammed gives 'SLIGHT'.
(Other definitions for slight that I've seen before include "Flimsy, tenuous" , "of no great significance" , "Minor - insult" , "cut" , "Insult, affront" .)