Why arson is wrong about the end of June (6)
I believe the answer is:
reason
'why' is the definition.
(thesaurus)
'arson is wrong about the end of june' is the wordplay.
'is wrong' is an anagram indicator.
'about' is an insertion indicator.
'the end of' says to take the final letters.
The final letter of 'june' is 'e'.
'arson' is an anagram of 'rason'.
'rason' placed around 'e' is 'REASON'.
(Other definitions for reason that I've seen before include "Explanation for some event" , "Cause; mind's power" , "Motivation or cause" , "Explanation or motive" , "Cause; think logically" .)