For this reason monster is upset (4)
I believe the answer is:
ergo
'for this reason' is the definition.
('ergo' is a Latin word meaning 'therefore')
'monster is upset' is the wordplay.
'monster' becomes 'ogre' (fairy tale monster).
'is upset' is a reversal indicator (I've seen 'being upset' mean this).
'ogre' back-to-front is 'ERGO'.
(Other definitions for ergo that I've seen before include "Consequently" , "for this reason" , "so philosopher wrote in Latin" , "'Cogito, ---- sum'" , "Hence" .)