For this reason monster arises (4)
I believe the answer is:
ergo
'for this reason' is the definition.
(I've seen this before)
'monster arises' is the wordplay.
'monster' becomes 'ogre' (fairy tale monster).
'arises' says the letters should be written in reverse (I've seen 'arising' mean this).
'ogre' in reverse letter order is 'ERGO'.
(Other definitions for ergo that I've seen before include "so philosopher wrote in Latin" , "'Therefore, to the old Romans (4)'" , "as a consequence" , "Thus" , "for this reason" .)