Why arson is terrible out East (6)
I believe the answer is:
reason
'why' is the definition.
(I've seen this in another clue)
'arson is terrible out east' is the wordplay.
'is terrible' indicates an anagram.
'out' means one lot of letters goes inside another.
'east' becomes 'e' (abbreviation).
'arson' anagrammed gives 'rason'.
'rason' placed around 'e' is 'REASON'.
(Other definitions for reason that I've seen before include "Sanity of senora" , "Cause, justification" , "Purpose" , "Think in a logical way" , "Employ logic" .)