So back “Bogeyman” (4)
I believe the answer is:
ergo
'so' is the definition.
('ergo' can be a synonym of 'so')
'back bogeyman' is the wordplay.
'back' is a reversal indicator.
'bogeyman' becomes 'ogre' (I've seen this before).
'ogre' written backwards gives 'ERGO'.
(Other definitions for ergo that I've seen before include "Thus" , "Therefore - what a goer" , "as a consequence" , "for this reason" , "Consequently" .)