Motive for terrible arson out East (6)
I believe the answer is:
reason
'motive' is the definition.
(I know that reason is a type of rational motive)
'terrible arson out east' is the wordplay.
'terrible' is an anagram indicator.
'out' means one lot of letters goes inside another.
'east' becomes 'e' (abbreviation).
'arson' with letters rearranged gives 'rason'.
'rason' enclosing 'e' is 'REASON'.
'for' acts as a link.
(Other definitions for reason that I've seen before include "logical argument" , "issue" , "Logic, sense" , "Mental power to justify an act" , "Justification; sanity" .)