Turn back on monster (4)
I believe the answer is:
ogre
'monster' is the definition.
('ogre' can be a synonym of 'monster')
'turn back on' is the wordplay.
'turn' becomes 'go' (a person's move in a board game).
'back' says the letters should be written in reverse.
'on' becomes 're' ('on' can be mean 'with reference to').
'go' written backwards gives 'og'.
'og'+'re'='OGRE'
(Other definitions for ogre that I've seen before include "'Demon, fiend (4)'" , "stern one who inspires fear" , "Terrifying man" , "Terrifying person" , "Giant of folklore, eats humans" .)