Motive for arson, perhaps, out East (6)
I believe the answer is:
reason
'motive' is the definition.
(reason is a kind of motive)
'arson perhaps out east' is the wordplay.
'perhaps' indicates anagramming the letters.
'out' indicates putting letters inside.
'east' becomes 'e' ('E' can be a synonym of 'east').
'arson' anagrammed gives 'rason'.
'rason' enclosing 'e' is 'REASON'.
'for' acts as a link.
(Other definitions for reason that I've seen before include "Rational motive" , "Cause; mind's power" , "Cause or motive" , "use of wit" , "Sanity and logic" .)