Why arson is wrong about the end of June (6)
I believe the answer is:
reason
'why' is the definition.
(I've seen this in another clue)
'arson is wrong about the end of june' is the wordplay.
'is wrong' indicates an anagram.
'about' means one lot of letters goes inside another.
'the end of' says to take the final letters.
The last 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" .)