Go back again to the monster (4)
I believe the answer is:
ogre
'the monster' is the definition.
(fairy tale monster)
'go back again' is the wordplay.
'back' is a reversal indicator.
'again' becomes 're' (prefix meaning 'again').
'go' back-to-front is 'og'.
'og'+'re'='OGRE'
'to' acts as a link.
(Other definitions for ogre that I've seen before include "Bogeyman" , "Ugly giant" , "Terrifying monster" , "Terrifying giant" , "ugly one" .)